Liquid crystal display device

ABSTRACT

It is an object to provide a liquid crystal display device which has excellent viewing angle characteristics and higher quality. The present invention has a pixel including a first switch, a second switch, a third switch, a first resistor, a second resistor, a first liquid crystal element, and a second liquid crystal element. A pixel electrode of the first liquid crystal element is electrically connected to a signal line through the first switch. The pixel electrode of the first liquid crystal element is electrically connected to a pixel electrode of the second liquid crystal element through the second switch and the first resistor. The pixel electrode of the second liquid crystal element is electrically connected to a Cs line through the third switch and the second resistor. A common electrode of the first liquid crystal element is electrically connected to a common electrode of the second liquid crystal element.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.13/793,427, filed Mar. 11, 2013, now allowed, which is a continuation ofU.S. application Ser. No. 13/399,147, filed Feb. 17, 2012, now U.S. Pat.No. 8,395,718, which is a continuation of U.S. application Ser. No.13/174,842, filed Jul. 1, 2011, now U.S. Pat. No. 8,120,721, which is adivisional of U.S. application Ser. No. 12/118,982, filed May 12, 2008,now U.S. Pat. No. 7,978,277, which claims the benefit of a foreignpriority application filed in Japan as Serial No. 2007-132172 on May 17,2007, all of which are incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an object, a method, or a method toproduce an object. The present invention particularly relates to adisplay device or a semiconductor device, and further relates to anelectronic device including the display device in a display portion.

2. Description of the Related Art

Liquid crystal display devices are used for many kinds of electronicdevices such as a mobile telephone, a television receiver, or the like,and many researches are done for further improving the quality.

While advantage of a liquid crystal display device is small size, lightweight, and low power consumption compared to a CRT (cathode-ray tube),the problem of the liquid crystal display device is the narrow viewingangle. In recent years, many researches about a multi domain method,that is, an alignment division method are made for improvingcharacteristics of viewing angle. For example, an MVA (multi-domainvertical alignment) mode which is combination of a VA (verticalalignment) mode and a multi-domain mode, a PVA (patterned verticalalignment) mode, and the like can be given.

In addition, researches are made in which one pixel is divided into aplurality of subpixels and the alignment state of liquid crystal in eachsubpixel is made different so as to improve the viewing anglecharacteristics (Reference 1: Japanese Published Patent Application No.2006-276582).

SUMMARY OF THE INVENTION

However, viewing angle characteristics are not enough, and furtherimprovement is required as a display device. In view of this situation,it is an object of the present invention to provide a higher qualitydisplay device having excellent viewing angle characteristics.

FIG. 84 is an equivalent circuit diagram of one pixel included in aliquid crystal display device described in Reference 1. The pixelillustrated in FIG. 84 includes a TFT 30, a liquid crystal capacitanceL₁, a liquid crystal capacitance L₂, a storage capacitor C₁, and astorage capacitor C₂, and is connected to a scan line 3 a and a dataline (signal line) 6 a.

Although liquid crystal has a voltage holding property, the holding rateis not 100%, and there is concern about a leak by charge passing toliquid crystal. When the leak occurs in the pixel shown in FIG. 84, aconstant potential state is lost by the law of conservation of charge ata node 11 which is closed by the storage capacitor C₁, the storagecapacitor C₂, and the liquid crystal capacitance L₂, and the potentialof the node 11 becomes far different from the initial state which isbefore the leak occurs. Therefore, the transmittance of the liquidcrystal capacitance L₂ changes from the transmittance which isdetermined by the image signal written in the data line 6 a, that is,the potential based on the gray scale. As a result, desired gray scalecan not be obtained, or the image quality is degraded. Thus, the displayquality is gradually degraded in course of time, so that the life ofproducts is shortened. Specifically, a display device of a normallyblack mode can not display black color, which leads to low contrast.

In view of the above mentioned problems, it is an object of the presentinvention to realize wide viewing angle display. Alternatively, it isanother object to provide a display device which is excellent incontrast. Alternatively, it is another object to provide a displaydevice higher display quality. Alternatively, it is another object toprovide a display device which is less likely to be influenced by noiseand can perform clear display. Alternatively, it is another object toprovide a display device in which display degradation is less likely tobe caused. Alternatively, it is another object to provide a displaydevice which has the long life time.

One aspect of the present invention is a liquid crystal display deviceincluding a pixel which has a first switch and a second switchcontrolled by a first scan line, a third switch controlled by the firstscan line and a second scan line, a first resistor, a second resistor, afirst liquid crystal element, and a second liquid crystal element. Eachof the first liquid crystal element and the second liquid crystalelement includes at least a pixel electrode, a common electrode, and aliquid crystal controlled by the pixel electrode and the commonelectrode, the pixel electrode of the first liquid crystal element iselectrically connected to a first wiring through the first switch. Thepixel electrode of the first liquid crystal element is electricallyconnected to the pixel electrode of the second liquid crystal elementthrough the second switch and the first resistor. The pixel electrode ofthe second liquid crystal element is electrically connected to a secondwiring through the third switch and the second resistor. The commonelectrode of the first liquid crystal element is electrically connectedto the common electrode of the second liquid crystal element.

Another aspect of the present invention is a liquid crystal displaydevice including a pixel which has a first switch and a second switchcontrolled by a first scan line, a third switch controlled by the firstscan line and a second scan line, a first resistor, a second resistor, afirst liquid crystal element, a second liquid crystal element, a firststorage capacitor, and a second storage capacitor. Each of the firstliquid crystal element and the second liquid crystal element includes atleast a pixel electrode, a common electrode, and a liquid crystalcontrolled by the pixel electrode and the common electrode. The pixelelectrode of the first liquid crystal element is electrically connectedto a first wiring through the first switch. The pixel electrode of thefirst liquid crystal element is electrically connected to the pixelelectrode of the second liquid crystal element through the second switchand the first resistor. The pixel electrode of the second liquid crystalelement is electrically connected to the second wiring through the thirdswitch and the second resistor. The pixel electrode of the first liquidcrystal element is electrically connected to the second wiring throughthe first storage capacitor. The pixel electrode of the second liquidcrystal element is electrically connected to the second wiring throughthe second storage capacitor. The common electrode of the first liquidcrystal element is electrically connected to the common electrode of thesecond liquid crystal element.

Another aspect of the present invention is a liquid crystal displaydevice including a pixel which has a first switch controlled by a thirdscan line, a second switch controlled by a first scan line, a thirdswitch controlled by the first scan line and a second scan line, a firstresistor, a second resistor, a first liquid crystal element, and secondliquid crystal element. Each of the first liquid crystal element and thesecond liquid crystal element includes at least a pixel electrode, acommon electrode, and a liquid crystal controlled by the pixel electrodeand the common electrode. The pixel electrode of the first liquidcrystal element is electrically connected to a first wiring through thefirst switch. The pixel electrode of the first liquid crystal element iselectrically connected to the pixel electrode of the second liquidcrystal element through the second switch and the first resistor. Thepixel electrode of the second liquid crystal element is electricallyconnected to a second wiring through the third switch and the secondresistor. The common electrode of the first liquid crystal element iselectrically connected to the common electrode of the second liquidcrystal element.

Another aspect of the present invention is a liquid crystal displaydevice including a pixel which has a first switch controlled by a thirdscan line, a second switch controlled by a first scan line, a thirdswitch controlled by the first scan line and a second scan line, a firstresistor, a second resistor, a first liquid crystal element, a secondliquid crystal element, a first storage capacitor, and a second storagecapacitor. Each of the first liquid crystal element and the secondliquid crystal element includes at least a pixel electrode, a commonelectrode, and a liquid crystal controlled by the pixel electrode andthe common electrode. The pixel electrode of the first liquid crystalelement is electrically connected to a first wiring through the firstswitch. The pixel electrode of the first liquid crystal element iselectrically connected to the pixel electrode of the second liquidcrystal element through the second switch and the first resistor. Thepixel electrode of the second liquid crystal element is electricallyconnected to a second wiring through the third switch and the secondresistor. The pixel electrode of the first liquid crystal element iselectrically connected to the second wiring through the first storagecapacitor. The pixel electrode of the second liquid crystal element iselectrically connected to the second wiring through the second storagecapacitor. The common electrode of the first liquid crystal element iselectrically connected to the common electrode of the second liquidcrystal element.

Another aspect of the present invention is a liquid crystal displaydevice in which resistance of the second resistor is higher than that ofthe first resistor in the foregoing structure.

Another aspect of the present invention is a liquid crystal displaydevice including a pixel which has a switch controlled by a first scanline, a first transistor, a second transistor, a first liquid crystalelement, a second liquid crystal element, a first storage capacitor, anda second storage capacitor. A gate electrode of the first transistor iselectrically connected to the first scan line. The second transistorincludes a third transistor of which gate electrode is electricallyconnected to the first scan line and a fourth transistor of which gateelectrode is electrically connected to the second scan line, and thethird transistor and the fourth transistor are connected in series. Eachof the first liquid crystal element and the second liquid crystalelement includes at least a pixel electrode, a common electrode, and aliquid crystal controlled by the pixel electrode and the commonelectrode. The pixel electrode of the first liquid crystal element iselectrically connected to a first wiring through the switch. The pixelelectrode of the first liquid crystal element is electrically connectedto the pixel electrode of the second liquid crystal element through thefirst transistor. The pixel electrode of the second liquid crystalelement is electrically connected to a second wiring through the secondtransistor. The pixel electrode of the first liquid crystal element iselectrically connected to the second wiring through the first storagecapacitor. The pixel electrode of the second liquid crystal element iselectrically connected to the second wiring through the second storagecapacitor. The common electrode of the first liquid crystal element iselectrically connected to the common electrode of the second liquidcrystal element.

Another aspect of the present invention is a liquid crystal displaydevice including a pixel which has a first transistor, a secondtransistor, a third transistor, a first liquid crystal element, a secondliquid crystal element, a first storage capacitor, and a second storagecapacitor. A gate electrode of the first transistor and a gate electrodeof the third transistor are electrically connected to a first scan line.The second transistor includes a fourth transistor of which gateelectrode is electrically connected to the first scan line and a fifthtransistor of which gate electrode is electrically connected to thesecond scan line, and the fourth transistor and the fifth transistor areconnected in series. Each of the first liquid crystal element and thesecond liquid crystal element includes at least a pixel electrode, acommon electrode, and a liquid crystal controlled by the pixel electrodeand the common electrode. The pixel electrode of the first liquidcrystal element is electrically connected to a first wiring through thethird transistor. The pixel electrode of the first liquid crystalelement is electrically connected to the pixel electrode of the secondliquid crystal element through the first transistor. The pixel electrodeof the second liquid crystal element is electrically connected to asecond wiring through the second transistor. The pixel electrode of thefirst liquid crystal element is electrically connected to the secondwiring through the first storage capacitor. The pixel electrode of thesecond liquid crystal element is electrically connected to the secondwiring through the second storage capacitor. The common electrode of thefirst liquid crystal element is electrically connected to the commonelectrode of the second liquid crystal element.

Another aspect of the present invention is the liquid crystal displaydevice, in which, when W represents the channel width of a transistorand L represents the channel length of a transistor, W/L of the thirdtransistor is smaller than W/L of the first transistor or the secondtransistor in the foregoing structure.

Another aspect of the present invention is in the liquid crystal displaydevice, in which, when W represents the channel width of a transistorand L represents channel length of a transistor, W/L of the secondtransistor is larger than W/L of the first transistor in the foregoingstructure.

Note that as the display device of the present invention, an activematrix display device such as a liquid crystal display device, alight-emitting device provided with a light-emitting element typified byan organic light-emitting element (OLED) in each pixel, a DMD (digitalmicromirror device), a PDP (plasma display panel), or an FED (fieldemission display) is included in its category. In addition, a passivematrix display device is included in its category.

Note that various types of switches can be used as a switch. Anelectrical switch, a mechanical switch, and the like are given asexamples. That is, any element can be used as long as it can control acurrent flow, without limiting to a certain element. For example, atransistor (e.g., a bipolar transistor or a MOS transistor), a diode(e.g., a PN diode, a PIN diode, a Schottky diode, an MIM (metalinsulator metal) diode, an MIS (metal insulator semiconductor) diode, ora diode-connected transistor), a thyristor, or the like can be used as aswitch. Alternatively, a logic circuit combining such elements can beused as a switch.

An example of a mechanical switch is a switch formed using MEMS (microelectro mechanical system) technology, such as a digital micromirrordevice (DMD). Such a switch includes an electrode which can be movedmechanically, and operates by controlling connection and non-connectionbased on movement of the electrode.

In the case of using a transistor as a switch, polarity (a conductivitytype) of the transistor is not particularly limited because it operatesjust as a switch. However, a transistor of polarity with smalleroff-current is preferably used when off-current is to be suppressed.Examples of a transistor with smaller off-current are a transistorprovided with an LDD region, a transistor with a multi-gate structure,and the like. In addition, it is preferable that an N-channel transistorbe used when a potential of a source terminal is closer to a potentialof a low-potential-side power supply (e.g., Vss, GND, or 0 V), while aP-channel transistor be used when the potential of the source terminalis closer to a potential of a high-potential-side power supply (e.g.,Vdd). This is because the absolute value of gate-source voltage can beincreased when the potential of the source terminal is closer to apotential of a low-potential-side power supply in an N-channeltransistor and when the potential of the source terminal is closer to apotential of a high-potential-side power supply in a P-channeltransistor, so that the transistor can be easily operated as a switch.In addition, this is also because the transistor does not often performa source follower operation, so that reduction in output voltage doesnot often occur.

Note that a CMOS switch may be used as a switch by using both N-channeland P-channel transistors. When a CMOS switch is used, the switch canmore precisely operate as a switch because current can flow when eitherthe P-channel transistor or the N-channel transistor is turned on. Forexample, voltage can be appropriately output regardless of whethervoltage of an input signal to the switch is high or low. In addition,since a voltage amplitude value of a signal for turning on or off theswitch can be made smaller, power consumption can be reduced.

Note that when a transistor is used as a switch, the switch includes aninput terminal (one of a source terminal and a drain terminal), anoutput terminal (the other of the source terminal and the drainterminal), and a terminal for controlling conduction (a gate terminal).On the other hand, when a diode is used as a switch, the switch does nothave a terminal for controlling conduction in some cases. Therefore,when a diode is used as a switch, the number of wirings for controllingterminals can be further reduced compared to the case of using atransistor as a switch.

Note that when it is explicitly described that “A and B are connected”,the case where A and B are electrically connected, the case where A andB are functionally connected, and the case where A and B are directlyconnected are included therein. Here, each of A and B corresponds to anobject (e.g., a device, an element, a circuit, a wiring, an electrode, aterminal, a conductive film, or a layer). Accordingly, another elementmay be interposed between elements having a connection relation shown indrawings and texts, without limiting to a predetermined connectionrelation, for example, the connection relation shown in the drawings andthe texts.

For example, in the case where A and B are electrically connected, oneor more elements which enable electric connection between A and B (e.g.,a switch, a transistor, a capacitor, an inductor, a resistor, and/or adiode) may be provided between A and B. In addition, in the case where Aand B are functionally connected, one or more circuits which enablefunctional connection between A and B (e.g., a logic circuit such as aninverter, a NAND circuit, or a NOR circuit, a signal converter circuitsuch as a DA converter circuit, an AD converter circuit, or a gammacorrection circuit, a potential level converter circuit such as a powersupply circuit (e.g., a dc-dc converter, a step-up dc-dc converter, or astep-down dc-dc converter) or a level shifter circuit for changing apotential level of a signal, a voltage source, a current source, aswitching circuit, or an amplifier circuit such as a circuit which canincrease signal amplitude, the amount of current, or the like (e.g., anoperational amplifier, a differential amplifier circuit, a sourcefollower circuit, or a buffer circuit), a signal generating circuit, amemory circuit, and/or a control circuit) may be provided between A andB. Alternatively, in the case where A and B are directly connected, Aand B may be directly connected without interposing another element oranother circuit therebetween.

Note that when it is explicitly described that “A and B are directlyconnected”, the case where A and B are directly connected (i.e., thecase where A and B are connected without interposing another element oranother circuit therebetween) and the case where A and B areelectrically connected (i.e., the case where A and B are connected byinterposing another element or another circuit therebetween) areincluded therein.

Note that when it is explicitly described that “A and B are electricallyconnected”, the case where A and B are electrically connected (i.e., thecase where A and B are connected by interposing another element oranother circuit therebetween), the case where A and B are functionallyconnected (i.e.; the case where A and B are functionally connected byinterposing another circuit therebetween), and the case where A and Bare directly connected (i.e., the case where A and B are connectedwithout interposing another element or another circuit therebetween) areincluded therein. That is, when it is explicitly described that “A and Bare electrically connected”, the description is the same as the casewhere it is explicitly only described that “A and B are connected”.

Note that a display element, a display device which is a device having adisplay element, a light-emitting element, and a light-emitting devicewhich is a device having a light-emitting element can use various typesand can include various elements. For example, a display medium, whosecontrast, luminance, reflectivity, transmittivity, or the like changesby an electromagnetic action, such as an EL element (e.g., an EL elementincluding organic and inorganic materials, an organic EL element, or aninorganic EL element), an electron emitter, a liquid crystal element,electronic ink, an electrophoresis element, a grating light valve (GLV),a plasma display panel (PDP), a digital micromirror device (DMD), apiezoelectric ceramic display, or a carbon nanotube can be used as adisplay element, a display device, a light-emitting element, or alight-emitting device. Note that display devices using an EL elementinclude an EL display; display devices using an electron emitter includea field emission display (FED), an SED-type flat panel display (SED:surface-conduction electron-emitter display), and the like; displaydevices using a liquid crystal element include a liquid crystal display(e.g., a transmissive liquid crystal display, a transflective liquidcrystal display, a reflective liquid crystal display, a direct-viewliquid crystal display, or a projection liquid crystal display); anddisplay devices using electronic ink or an electrophoresis elementinclude electronic paper.

Note that an EL element is an element having an anode, a cathode, and anEL layer interposed between the anode and the cathode. Note that as anEL layer, a layer utilizing light emission (fluorescence) from a singletexciton, a layer utilizing light emission (phosphorescence) from atriplet exciton, a layer utilizing light emission (fluorescence) from asinglet exciton and light emission (phosphorescence) from a tripletexciton, a layer formed of an organic material, a layer formed of aninorganic material, a layer formed of an organic material and aninorganic material, a layer including a high-molecular material, a layerincluding a low molecular material, a layer including a low-molecularmaterial and a high-molecular material, or the like can be used. Notethat the present invention is not limited to this, and various ELelements can be used as an EL element.

Note that an electron emitter is an element in which electrons areextracted by high electric field concentration on a pointed cathode. Forexample, as an electron emitter, a Spindt type, a carbon nanotube (CNT)type, a metal-insulator-metal (MIM) type in which a metal, an insulator,and a metal are stacked, a metal-insulator-semiconductor (MIS) type inwhich a metal, an insulator, and a semiconductor are stacked, a MOStype, a silicon type, a thin film diode type, a diamond type, a surfaceconduction emitter SCD type, a thin film type in which a metal, aninsulator, a semiconductor, and a metal are stacked, a HEED type, an ELtype, a porous silicon type, a surface-conduction (SED) type, or thelike can be used. However, the present invention is not limited to this,and various elements can be used as an electron emitter.

Note that a liquid crystal element is an element which controlstransmission or non-transmission of light by optical modulation actionof a liquid crystal and includes a pair of electrodes and a liquidcrystal. Note that optical modulation action of a liquid crystal iscontrolled by an electric filed applied to the liquid crystal (includinga horizontal electric field, a vertical electric field, and an obliqueelectric field). Note that the following can be used for a liquidcrystal element: a nematic liquid crystal, a cholesteric liquid crystal,a smectic liquid crystal, a discotic liquid crystal, a thermotropicliquid crystal, a lyotropic liquid crystal, a low-molecular liquidcrystal, a high-molecular liquid crystal, a ferroelectric liquidcrystal, an anti-ferroelectric liquid crystal, a main-chain liquidcrystal, a side-chain high-molecular liquid crystal, a plasma addressedliquid crystal (PALC), a banana-shaped liquid crystal, and the like. Inaddition, the following can be used as a diving method of a liquidcrystal: a TN (twisted nematic) mode, an STN (super twisted nematic)mode, an IPS (in-plane-switching) mode, an FFS (fringe field switching)mode, an MVA (multi-domain vertical alignment) mode, a PVA (patternedvertical alignment) mode, an ASV (advanced super view) mode, an ASM(axially symmetric aligned microcell) mode, an OCB (optical compensatedbirefringence) mode, an ECB (electrically controlled birefringence)mode, an FLC (ferroelectric liquid crystal) mode, an AFLC(anti-ferroelectric liquid crystal) mode, a PDLC (polymer dispersedliquid crystal) mode, a guest-host mode, and the like. Note that thepresent invention is not limited to this, and various liquid crystalelements and driving methods can be used as a liquid crystal element anda driving method thereof.

Note that electronic paper corresponds to a device which displays animage by molecules which utilize optical anisotropy, dye molecularorientation, or the like; a device which displays an image by particleswhich utilize electrophoresis, particle movement, particle rotation,phase change, or the like; a device which displays an image by movingone end of a film; a device which displays an image by using coloringproperties or phase change of molecules; a device which displays animage by using optical absorption by molecules; and a device whichdisplays an image by using self-light emission by bonding electrons andholes. For example, the following can be used for electronic paper:microcapsule electrophoresis, horizontal electrophoresis, verticalelectrophoresis, a spherical twisting ball, a magnetic twisting ball, acolumnar twisting ball, a charged toner, electro liquid powder, magneticelectrophoresis, a magnetic thermosensitive type, an electrowettingtype, a light-scattering (transparent-opaque change) type, a cholestericliquid crystal and a photoconductive layer, a cholesteric liquid crystaldevice, a bistable nematic liquid crystal, a ferroelectric liquidcrystal, a liquid crystal dispersed type with a dichroic dye, a movablefilm, coloring and decoloring properties of a leuco dye, a photochromicmaterial, an electrochromic material, an electro deposition material,flexible organic EL, and the like. Note that the present invention isnot limited to this, and a variety of electronic paper can be used aselectronic paper. Here, when microcapsule electrophoresis is used,defects of electrophoresis, which are aggregation and precipitation ofphoresis particles, can be solved. Electro liquid powder has advantagessuch as high-speed response, high reflectivity, wide viewing angle, lowpower consumption, and memory properties.

Note that a plasma display has a structure in which a substrate having asurface provided with an electrode and a substrate having a surfaceprovided with an electrode and a minute groove in which a phosphor layeris formed face each other at a narrow interval and a rare gas is sealedtherein. Note that display can be performed by applying voltage betweenthe electrodes to generate an ultraviolet ray so that a phosphor emitslight. Note that the plasma display may be a DC-type PDP or an AC-typePDP. For the plasma display, AWS (address while sustain) driving, ADS(address display separated) driving in which a subframe is divided intoa reset period, an address period, and a sustain period, CLEAR (lowenergy address and reduction of false contour sequence) driving, ALIS(alternate lighting of surfaces) method, TERES (technology of reciprocalsustainer) driving, or the like can be used. Note that the presentinvention is not limited to this, and various plasma displays can beused as a plasma display panel.

Note that electroluminescence, a cold cathode fluorescent lamp, a hotcathode fluorescent lamp, an LED, a laser light source, a mercury lamp,or the like can be used as a light source of a display device in which alight source is necessary, such as a liquid crystal display (atransmissive liquid crystal display, a transflective liquid crystaldisplay, a reflective liquid crystal display, a direct-view liquidcrystal display, or a projection liquid crystal display), a displaydevice using a grating light valve (GLV), or a display device using adigital micromirror device (DMD). Note that the present invention is notlimited to this, and various light sources can be used as a lightsource.

Note that various types of transistors can be used as a transistor,without limiting to a certain type. For example, a thin film transistor(TFT) including a non-single crystal semiconductor film typified byamorphous silicon, polycrystalline silicon, microcrystalline (alsoreferred to as semi-amorphous) silicon, or the like can be used. In thecase of using the TFT, there are various advantages. For example, sincethe TFT can be formed at temperature lower than that of the case ofusing single-crystal silicon, manufacturing cost can be reduced or amanufacturing apparatus can be made larger. Since the manufacturingapparatus is made larger, the TFT can be formed using a large substrate.Therefore, many display devices can be formed at the same time at lowcost. In addition, a substrate having low heat resistance can be usedbecause of low manufacturing temperature. Therefore, the transistor canbe formed using a light-transmitting substrate. Accordingly,transmission of light in a display element can be controlled by usingthe transistor formed using the light-transmitting substrate.Alternatively, part of a film which forms the transistor can transmitlight because the film thickness of the transistor is thin. Therefore,the aperture ratio can be improved.

Note that when a catalyst (e.g., nickel) is used in the case of formingpolycrystalline silicon, crystallinity can be further improved and atransistor having excellent electric characteristics can be fat ed.Accordingly, a gate driver circuit (e.g., a scan line driver circuit), asource driver circuit (e.g., a signal line driver circuit), and/or asignal processing circuit (e.g., a signal generation circuit, a gammacorrection circuit, or a DA converter circuit) can be formed over thesame substrate as a pixel portion.

Note that when a catalyst (e.g., nickel) is used in the case of formingmicrocrystalline silicon, crystallinity can be further improved and atransistor having excellent electric characteristics can be formed. Atthis time, crystallinity can be improved by just performing heattreatment without performing laser light irradiation. Accordingly, agate driver circuit (e.g., a scan line driver circuit) and part of asource driver circuit (e.g., an analog switch) can be formed over thesame substrate. In addition, in the case of not performing laser lightirradiation for crystallization, crystallinity unevenness of silicon canbe suppressed. Therefore, a clear image can be displayed.

Note that polycrystalline silicon and microcrystalline silicon can beformed without using a catalyst (e.g., nickel).

Note that it is preferable that crystallinity of silicon be improved topolycrystal, microcrystal, or the like in the whole panel; however, thepresent invention is not limited to this. Crystallinity of silicon maybe improved only in part of the panel. Selective increase incrystallinity can be achieved by selective laser irradiation or thelike. For example, only a peripheral driver circuit region excludingpixels may be irradiated with laser light. Alternatively, only a regionof a gate driver circuit, a source driver circuit, or the like may beirradiated with laser light. Further alternatively, only part of asource driver circuit (e.g., an analog switch) may be irradiated withlaser light. Accordingly, crystallinity of silicon can be improved onlyin a region in which a circuit needs to be operated at high speed. Sincea pixel region is not particularly needed to be operated at high speed,even if crystallinity is not improved, the pixel circuit can be operatedwithout problems. Since a region, crystallinity of which is improved, issmall, manufacturing steps can be decreased, throughput can beincreased, and manufacturing cost can be reduced. Since the number ofnecessary manufacturing apparatus is small, manufacturing cost can bereduced.

A transistor can be formed by using a semiconductor substrate, an SOIsubstrate, or the like. Thus, a transistor with few variations incharacteristics, sizes, shapes, or the like, with high current supplycapacity, and with a small size can be formed. When such a transistor isused, power consumption of a circuit can be reduced or a circuit can behighly integrated.

A transistor including a compound semiconductor or an oxidesemiconductor such as ZnO, a-InGaZnO, SiGe, GaAs, IZO, ITO, or SnO, athin film transistor obtained by thinning such a compound semiconductoror an oxide semiconductor, or the like can be used. Thus, manufacturingtemperature can be lowered and for example, such a transistor can beformed at room temperature. Accordingly, the transistor can be formeddirectly on a substrate having low heat resistance, such as a plasticsubstrate or a film substrate. Note that such a compound semiconductoror an oxide semiconductor can be used for not only a channel portion ofthe transistor but also other applications. For example, such a compoundsemiconductor or an oxide semiconductor can be used as a resistor, apixel electrode, or a light-transmitting electrode. Further, since suchan element can be formed at the same time as the transistor, cost can bereduced.

A transistor formed by using an inkjet method or a printing method, orthe like can be used. Accordingly, a transistor can be formed at roomtemperature, can be formed at a low vacuum, or can be formed using alarge substrate. In addition, since the transistor can be formed withoutusing a mask (a reticle), a layout of the transistor can be easilychanged. Further, since it is not necessary to use a resist, materialcost is reduced and the number of steps can be reduced. Furthermore,since a film is formed only in a necessary portion, a material is notwasted compared with a manufacturing method in which etching isperformed after the film is formed over the entire surface, so that costcan be reduced.

A transistor including an organic semiconductor or a carbon nanotube, orthe like can be used. Accordingly, such a transistor can be formed usinga substrate which can be bent. Therefore, such a device can resist ashock.

Further, transistors with various structures can be used. For example, aMOS transistor, a junction transistor, a bipolar transistor, or the likecan be used as a transistor. When a MOS transistor is used, the size ofthe transistor can be reduced. Thus, a large number of transistors canbe mounted. When a bipolar transistor is used, large current can flow.Thus, a circuit can be operated at high speed.

Note that a MOS transistor, a bipolar transistor, and the like may beformed over one substrate. Thus, reduction in power consumption,reduction in size, high speed operation, and the like can be realized.

Furthermore, various transistors can be used.

Note that a transistor can be formed using various types of substrateswithout limiting to a certain type. For example, a single-crystalsubstrate, an SOI substrate, a glass substrate, a quartz substrate, aplastic substrate, a paper substrate, a cellophane substrate, a stonesubstrate, a wood substrate, a cloth substrate (including a naturalfiber (e.g., silk, cotton, or hemp), a synthetic fiber (e.g., nylon,polyurethane, or polyester), a regenerated fiber (e.g., acetate, cupra,rayon, or regenerated polyester), or the like), a leather substrate, arubber substrate, a stainless steel substrate, a substrate including astainless steel foil, or the like can be used as a substrate over whicha transistor is formed. Alternatively, a skin (e.g., epidermis orcorium) or hypodermal tissue of an animal such as a human being can beused as a substrate. Further alternatively, the transistor may be formedusing one substrate, and then, the transistor may be transferred toanother substrate, and the transistor may be provided over anothersubstrate. A single-crystal substrate, an SOT substrate, a glasssubstrate, a quartz substrate, a plastic substrate, a paper substrate, acellophane substrate, a stone substrate, a wood substrate, a clothsubstrate (including a natural fiber (e.g., silk, cotton, or hemp), asynthetic fiber (e.g., nylon, polyurethane, or polyester), a regeneratedfiber (e.g., acetate, cupra, rayon, or regenerated polyester), or thelike), a leather substrate, a rubber substrate, a stainless steelsubstrate, a substrate including a stainless steel foil, or the like canbe used as a substrate to which the transistor is transferred.Alternatively, a skin (e.g., epidermis or corium) or hypodermal tissueof an animal such as a human being can be used as a substrate. Furtheralternatively, the transistor may be formed using one substrate and thesubstrate may be thinned by polishing. A single-crystal substrate, anSOT substrate, a glass substrate, a quartz substrate, a plasticsubstrate, a paper substrate, a cellophane substrate, a stone substrate,a wood substrate, a cloth substrate (including a natural fiber (e.g.,silk, cotton, or hemp), a synthetic fiber (e.g., nylon, polyurethane, orpolyester), a regenerated fiber (e.g., acetate, cupra, rayon, orregenerated polyester), or the like), a leather substrate, a rubbersubstrate, a stainless steel substrate, a substrate including astainless steel foil, or the like can be used as a substrate.Alternatively, a skin (e.g., epidermis or corium) or hypodermal tissueof an animal such as a human being can be used as a substrate to bepolished. When such a substrate is used, a transistor with excellentproperties or a transistor with low power consumption can be formed, adevice with high durability, high heat resistance can be provided, orreduction in weight or thickness can be achieved.

Note that a structure of a transistor can be various modes withoutlimiting to a certain structure. For example, a multi-gate structurehaving two or more gate electrodes may be used. When the multi-gatestructure is used, a structure where a plurality of transistors areconnected in series is provided because channel regions are connected inseries. With the multi-gate structure, off-current can be reduced or thewithstand voltage of the transistor can be increased to improvereliability. Alternatively, with the multi-gate structure, drain-sourcecurrent does not fluctuate very much even if drain-source voltagefluctuates when the transistor operates in a saturation region, so thata flat slope of voltage-current characteristics can be obtained. Whenthe flat slope of the voltage-current characteristics is utilized, anideal current source circuit or an active load having an extremely highresistance value can be realized. Accordingly, a differential circuit ora current mirror circuit having excellent properties can be realized. Inaddition, a structure where gate electrodes are formed above and below achannel may be used. When the structure where gate electrodes are formedabove and below the channel is used, a channel region is increased, sothat the amount of current flowing therethrough can be increased or adepletion layer can be easily formed to decrease subthreshold swing.When the gate electrodes are formed above and below the channel, astructure where a plurality of transistors are connected in parallel isprovided.

Alternatively, a structure where a gate electrode is formed above achannel region, a structure where a gate electrode is formed below achannel region, a staggered structure, an inversely staggered structure,a structure where a channel region is divided into a plurality ofregions, or a structure where channel regions are connected in parallelor in series can be used. Further alternatively, a source electrode or adrain electrode may overlap with a channel region (or part of it). Whenthe structure where the source electrode or the drain electrode mayoverlap with the channel region (or part of it) is used, the case can beprevented in which electric charges are accumulated in part of thechannel region, which would result in an unstable operation. Furtheralternatively, an LDD region may be provided. When the LDD region isprovided, off-current can be reduced or the withstand voltage of thetransistor can be increased to improve reliability. Further, when theLDD region is provided, drain-source current does not fluctuate verymuch even if drain-source voltage fluctuates when the transistoroperates in the saturation region, so that a flat slope ofvoltage-current characteristics can be obtained.

Note that various types of transistors can be used as a transistor andthe transistor can be formed using various types of substrates.Accordingly, all the circuits that are necessary to realize apredetermined function may be formed using the same substrate. Forexample, all the circuits that are necessary to realize thepredetermined function may be formed using a glass substrate, a plasticsubstrate, a single-crystal substrate, an SOI substrate, or any othersubstrate. When all the circuits that are necessary to realize thepredetermined function are formed using the same substrate, cost can bereduced by reduction in the number of component parts or reliability canbe improved by reduction in the number of connections to circuitcomponents. Alternatively, part of the circuits which are necessary torealize the predetermined function may be formed using one substrate andanother part of the circuits which are necessary to realize thepredetermined function may be formed using another substrate. That is,not all the circuits that are necessary to realize the predeterminedfunction are required to be faulted using the same substrate. Forexample, part of the circuits which are necessary to realize thepredetermined function may be formed by transistors using a glasssubstrate and another part of the circuits which are necessary torealize the predetermined function may be formed over a single-crystalsubstrate, so that an IC chip formed by a transistor over thesingle-crystal substrate may be connected to the glass substrate by COG(chip on glass) and the IC chip may be provided over the glasssubstrate. Alternatively, the IC chip may be connected to the glasssubstrate by TAB (tape automated bonding) or a printed wiring board.When part of the circuits are formed using the same substrate in thismanner, cost can be reduced by reduction in the number of componentparts or reliability can be improved by reduction in the number ofconnections to circuit components. Further alternatively, when circuitswith high driving voltage and high driving frequency, which consumelarge power, are formed over a single-crystal semiconductor substrateinstead of forming such circuits using the same substrate and an IC chipformed by the circuit is used, increase in power consumption can beprevented.

Note that one pixel corresponds to one element whose brightness can becontrolled. Therefore, for example, one pixel corresponds to one colorelement and brightness is expressed with the one color element.Accordingly, in the case of a color display device having color elementsof R (red), G (green), and B (blue), a minimum unit of an image isformed of three pixels of an R pixel, a G pixel, and a B pixel. Notethat the color elements are not limited to three colors, and colorelements of more than three colors may be used or a color other than RGBmay be used. For example, RGBW (W corresponds to white) may be used byadding white. Alternatively, one or more colors of yellow, cyan, magentaemerald green, vermilion, and the like may be added to RGB. Furtheralternatively, a color similar to at least one of R, G, and B may beadded to RGB. For example, R, G, B1, and B2 may be used. Although bothB1 and B2 are blue, they have slightly different frequency. Similarly,R1, R2, and B may be used. When such color elements are used, displaywhich is closer to the real object can be performed and powerconsumption can be reduced. As another example, in the case ofcontrolling brightness of one color element by using a plurality ofregions, one region may correspond to one pixel. Therefore, for example,in the case of performing area ratio gray scale display or the case ofincluding a subpixel, a plurality of regions which control brightnessare provided in each color element and gray scales are expressed withthe whole regions. In this case, one region which controls brightnessmay correspond to one pixel. Thus, in that case, one color elementincludes a plurality of pixels. Alternatively, even when the pluralityof regions which control brightness are provided in one color element,these regions may be collected as one pixel. Thus, in that case, onecolor element includes one pixel. In that case, one color elementincludes one pixel. Further alternatively, in the case where brightnessis controlled in a plurality of regions in each color element, regionswhich contribute to display have different area dimensions depending onpixels in some cases. Further alternatively, in the plurality of regionswhich control brightness in each color element, signals supplied to eachof the plurality of regions may be slightly varied to widen viewingangle characteristics. That is, potentials of pixel electrodes includedin the plurality of regions provided in each color element may bedifferent from each other. Accordingly, voltage applied to liquidcrystal molecules are varied depending on the pixel electrodes.Therefore, the viewing angle can be widened.

Note that explicit description “one pixel (for three colors)”corresponds to the case where three pixels of R, G, and B are consideredas one pixel. Meanwhile, explicit description “one pixel (for onecolor)” corresponds to the case where the plurality of regions areprovided in each color element and collectively considered as one pixel.

Note that in this document (the specification, the claim, the drawing,and the like), pixels are provided (arranged) in matrix in some cases.Here, description that pixels are provided (arranged) in matrix includesthe case where the pixels are arranged in a straight line and the casewhere the pixels are arranged in a jagged line, in a longitudinaldirection or a lateral direction. Thus, for example, in the case ofperforming full color display with three color elements (e.g., RGB), thefollowing cases are included therein: the case where the pixels arearranged in stripes and the case where dots of the three color elementsare arranged in a delta pattern. In addition, the case is also includedtherein in which dots of the three color elements are provided in Bayerarrangement. Note that the color elements are not limited to threecolors, and color elements of more than three colors may be used. Forexample, RGBW (W corresponds to white), RGB plus one or more of yellow,cyan, and magenta, or the like may be used. Further, the sizes ofdisplay regions may be different between respective dots of colorelements. Thus, power consumption can be reduced or the life of adisplay element can be prolonged.

Note that an active matrix method in which an active element is includedin a pixel or a passive matrix method in which an active element is notincluded in a pixel can be used.

In an active matrix method, as an active element (a non-linear element),not only a transistor but also various active elements (non-linearelements) can be used. For example, an MIM (metal insulator metal), aTFD (thin film diode), or the like can also be used. Since such anelement has few numbers of manufacturing steps, manufacturing cost canbe reduced or yield can be improved. Further, since the size of theelement is small, the aperture ratio can be improved, so that powerconsumption can be reduced or high luminance can be achieved.

Note that as a method other than an active matrix method, a passivematrix method in which an active element (a non-linear element) is notused can also be used. Since an active element (a non-linear element) isnot used, manufacturing steps is few, so that manufacturing cost can bereduced or the yield can be improved. Further, since an active element(a non-linear element) is not used, the aperture ratio can be improved,so that power consumption can be reduced or high luminance can beachieved.

Note that a transistor is an element having at least three terminals ofa gate, a drain, and a source. The transistor has a channel regionbetween a drain region and a source region, and current can flow throughthe drain region, the channel region, and the source region. Here, sincethe source and the drain of the transistor change depending on thestructure, the operating condition, and the like of the transistor, itis difficult to define which is a source or a drain. Therefore, in thisdocument (the specification, the claim, the drawing, and the like), aregion functioning as a source and a drain may not be called the sourceor the drain. In such a case, one of the source and the drain may bereferred to as a first terminal and the other thereof may be referred toas a second terminal, for example. Alternatively, one of the source andthe drain may be referred to as a first electrode and the other thereofmay be referred to as a second electrode. Further alternatively, one ofthe source and the drain may be referred to as a source region and theother thereof may be called a drain region.

Note that a transistor may be an element having at least three terminalsof a base, an emitter, and a collector. In this case, one of the emitterand the collector may be similarly referred to as a first terminal andthe other terminal may be referred to as a second terminal.

Note that a gate corresponds to all or part of a gate electrode and agate wiring (also referred to as a gate line, a gate signal line, a scanline, a scan signal line, or the like). A gate electrode corresponds toa conductive film which overlaps with a semiconductor which forms achannel region with a gate insulating film interposed therebetween. Notethat part of the gate electrode overlaps with an LDD (lightly dopeddrain) region or the source region (or the drain region) with the gateinsulating film interposed therebetween in some cases. A gate wiringcorresponds to a wiring for connecting a gate electrode of eachtransistor to each other, a wiring for connecting a gate electrode ofeach pixel to each other, or a wiring for connecting a gate electrode toanother wiring.

However, there is a portion (a region, a conductive film, a wiring, orthe like) which functions as both a gate electrode and a gate wiring.Such a portion (a region, a conductive film, a wiring, or the like) maybe referred to as either a gate electrode or a gate wiring. That is,there is a region where a gate electrode and a gate wiring cannot beclearly distinguished from each other. For example, in the case where achannel region overlaps with part of an extended gate wiring, theoverlapped portion (region, conductive film, wiring, or the like)functions as both a gate wiring and a gate electrode. Accordingly, sucha portion (a region, a conductive film, a wiring, or the like) may bereferred to as either a gate electrode or a gate wiring.

Note that a portion (a region, a conductive film, a wiring, or the like)which is formed using the same material as a gate electrode, forms thesame island as the gate electrode, and is connected to the gateelectrode may also be referred to as a gate electrode. Similarly, aportion (a region, a conductive film, a wiring, or the like) which isformed using the same material as a gate wiring, forms the same islandas the gate wiring, and is connected to the gate wiring may also bereferred to as a gate wiring. In a strict detect, such a portion (aregion, a conductive film, a wiring, or the like) does not overlap witha channel region or does not have a function of connecting the gateelectrode to another gate electrode in some cases. However, there is aportion (a region, a conductive film, a wiring, or the like) which isformed using the same material as a gate electrode or a gate wiring,forms the same island as the gate electrode or the gate wiring, and isconnected to the gate electrode or the gate wiring because ofspecifications or the like in manufacturing. Thus, such a portion (aregion, a conductive film, a wiring, or the like) may also be referredto as either a gate electrode or a gate wiring.

Note that in a multi-gate transistor, for example, a gate electrode isoften connected to another gate electrode by using a conductive filmwhich is formed using the same material as the gate electrode. Sincesuch a portion (a region, a conductive film, a wiring, or the like) is aportion (a region, a conductive film, a wiring, or the like) forconnecting the gate electrode to another gate electrode, it may bereferred to as a gate wiring, and it may also be referred to as a gateelectrode because a multi-gate transistor can be considered as onetransistor. That is, a portion (a region, a conductive film, a wiring,or the like) which is formed using the same material as a gate electrodeor a gate wiring, forms the same island as the gate electrode or thegate wiring, and is connected to the gate electrode or the gate wiringmay be referred to as either a gate electrode or a gate wiring. Inaddition, for example, part of a conductive film which connects the gateelectrode and the gate wiring and is formed using a material which isdifferent from that of the gate electrode or the gate wiring may also bereferred to as either a gate electrode or a gate wiring.

Note that a gate terminal corresponds to part of a portion (a region, aconductive film, a wiring, or the like) of a gate electrode or a portion(a region, a conductive film, a wiring, or the like) which iselectrically connected to the gate electrode.

Note that when a wiring is referred to as a gate wiring, a gate line, agate signal line, a scan line, a scan signal line, there is the case inwhich a gate of a transistor is not connected to a wiring. In this case,the gate wiring, the gate line, the gate signal line, the scan line, orthe scan signal line corresponds to a wiring formed in the same layer asthe gate of the transistor, a wiring formed using the same material ofthe gate of the transistor, or a wiring formed at the same time as thegate of the transistor in some cases. As examples, there are a wiringfor a storage capacitor, a power supply line, a reference potentialsupply line, and the like.

Note that a source corresponds to all or part of a source region, asource electrode, and a source wiring (also referred to as a sourceline, a source signal line, a data line, a data signal line, or thelike). A source region corresponds to a semiconductor region including alarge amount of p-type impurities (e.g., boron or gallium) or n-typeimpurities (e.g., phosphorus or arsenic). Therefore, a region includinga small amount of p-type impurities or n-type impurities, namely, an LDD(lightly doped drain) region is not included in the source region. Asource electrode is part of a conductive layer which is formed using amaterial different from that of a source region and is electricallyconnected to the source region. However, there is the case where asource electrode and a source region are collectively referred to as asource electrode. A source wiring is a wiring for connecting a sourceelectrode of each transistor to each other, a wiring for connecting asource electrode of each pixel to each other, or a wiring for connectinga source electrode to another wiring.

However, there is a portion (a region, a conductive film, a wiring, orthe like) functioning as both a source electrode and a source wiring.Such a portion (a region, a conductive film, a wiring, or the like) maybe referred to as either a source electrode or a source wiring. That is,there is a region where a source electrode and a source wiring cannot beclearly distinguished from each other. For example, in the case where asource region overlaps with part of an extended source wiring, theoverlapped portion (region, conductive film, wiring, or the like)functions as both a source wiring and a source electrode. Accordingly,such a portion (a region, a conductive film, a wiring, or the like) maybe referred to as either a source electrode or a source wiring.

Note that a portion (a region, a conductive film, a wiring, or the like)which is formed using the same material as a source electrode, forms thesame island as the source electrode, and is connected to the sourceelectrode, or a portion (a region, a conductive film, a wiring, or thelike) which connects a source electrode and another source electrode mayalso be referred to as a source electrode. Further, a portion whichoverlaps with a source region may be referred to as a source electrode.Similarly, a portion (a region, a conductive film, a wiring, or thelike) which is formed using the same material as a source wiring, formsthe same island as the source wiring, and is connected to the sourcewiring may also be referred to as a source wiring. In a strict sense,such a portion (a region, a conductive film, a wiring, or the like) doesnot have a function of connecting the source electrode to another sourceelectrode in some cases. However, there is a portion (a region, aconductive film, a wiring, or the like) which is formed using the samematerial as a source electrode or a source wiring, forms the same islandas the source electrode or the source wiring, and is connected to thesource electrode or the source wiring because of specifications or thelike in manufacturing. Thus, such a portion (a region, a conductivefilm, a wiring, or the like) may also be referred to as either a sourceelectrode or a source wiring.

For example, part of a conductive film which connects a source electrodeand a source wiring and is formed using a material which is differentfrom that of the source electrode or the source wiring may be referredto as either a source electrode or a source wiring.

Note that a source terminal corresponds to part of a source region, asource electrode, or a portion (a region, a conductive film, a wiring,or the like) which is electrically connected to the source electrode.

Note that when a wiring is referred to as a source wiring, a sourceline, a source signal line, a data line, a data signal line, there isthe case in which a source (a drain) of a transistor is not connected toa wiring. In this case, the source wiring, the source line, the sourcesignal line, the data line, or the data signal line corresponds to awiring formed in the same layer as the source (the drain) of thetransistor, a wiring formed using the same material of the source (thedrain) of the transistor, or a wiring formed at the same time as thesource (the drain) of the transistor in some cases. As examples, thereare a wiring for a storage capacitor, a power supply line, a referencepotential supply line, and the like.

Note that the same can be said for a drain.

Note that a semiconductor device corresponds to a device having acircuit including a semiconductor element (e.g., a transistor, a diode,or a thyristor). The semiconductor device may also include all devicesthat can function by utilizing semiconductor characteristics. Inaddition, the semiconductor device corresponds to a device having asemiconductor material.

Note that a display element corresponds to an optical modulationelement, a liquid crystal element, a light-emitting element, an ELelement (an organic EL element, an inorganic EL element, or an ELelement including organic and inorganic materials), an electron emitter,an electrophoresis element, a discharging element, a light-reflectiveelement, a light diffraction element, a digital micromirror device(DMD), or the like. Note that the present invention is not limited tothis.

Note that a display device corresponds to a device having a displayelement. The display device may include a plurality of pixels eachhaving a display element. Note that that the display device may alsoinclude a peripheral driver circuit for driving the plurality of pixels.The peripheral driver circuit for driving the plurality of pixels may beformed over the same substrate as the plurality of pixels. The displaydevice may also include a peripheral driver circuit provided over asubstrate by wire bonding or bump bonding, namely, an IC chip connectedby chip on glass (COG) or an IC chip connected by TAB or the like.Further, the display device may also include a flexible printed circuit(FPC) to which an IC chip, a resistor, a capacitor, an inductor, atransistor, or the like is attached. Note also that the display deviceincludes a printed wiring board (PWB) which is connected through aflexible printed circuit (FPC) and to which an IC chip, a resistor, acapacitor, an inductor, a transistor, or the like is attached. Thedisplay device may also include an optical sheet such as a polarizingplate or a retardation plate. The display device may also include alighting device, a housing, an audio input and output device, a lightsensor, or the like. Here, a lighting device such as a backlight unitmay include a light guide plate, a prism sheet, a diffusion sheet, areflective sheet, a light source (e.g., an LED or a cold cathodefluorescent lamp), a cooling device (e.g., a water cooling device or anair cooling device), or the like.

Note that a lighting device corresponds to a device having a backlightunit, a light guide plate, a prism sheet, a diffusion sheet, areflective sheet, or a light source (e.g., an LED, a cold cathodefluorescent lamp, or a hot cathode fluorescent lamp), a cooling device,or the like.

Note that a light-emitting device corresponds to a device having alight-emitting element and the like. In the case of including alight-emitting element as a display element, the light-emitting deviceis one of specific examples of a display device.

Note that a reflective device corresponds to a device having alight-reflective element, a light diffraction element, light-reflectiveelectrode, or the like.

Note that a liquid crystal display device corresponds to a displaydevice including a liquid crystal element. Liquid crystal displaydevices include a direct-view liquid crystal display, a projectionliquid crystal display, a transmissive liquid crystal display, areflective liquid crystal display, a transflective liquid crystaldisplay, and the like.

Note that a driving device corresponds to a device having asemiconductor element, an electric circuit, or an electronic circuit.For example, a transistor which controls input of a signal from a sourcesignal line to a pixel (also referred to as a selection transistor, aswitching transistor, or the like), a transistor which supplies voltageor current to a pixel electrode, a transistor which supplies voltage orcurrent to a light-emitting element, and the like are examples of thedriving device. A circuit which supplies a signal to a gate signal line(also referred to as a gate driver, a gate line driver circuit, or thelike), a circuit which supplies a signal to a source signal line (alsoreferred to as a source driver, a source line driver circuit, or thelike) are also examples of the driving device.

Note that a display device, a semiconductor device, a lighting device, acooling device, a light-emitting device, a reflective device, a drivingdevice, and the like overlap with each other in some cases. For example,a display device includes a semiconductor device and a light-emittingdevice in some cases. Alternatively, a semiconductor device includes adisplay device and a driving device in some cases.

Note that when it is explicitly described that “B is formed on A” or “Bis formed over A”, it does not necessarily mean that B is formed indirect contact with A. The description includes the case where A and Bare not in direct contact with each other, i.e., the case where anotherobject is interposed between A and B. Here, each of A and B correspondsto an object (e.g., a device, an element, a circuit, a wiring, anelectrode, a terminal, a conductive film, or a layer).

Accordingly, for example, when it is explicitly described that “a layerB is formed on (or over) a layer A”, it includes both the case where thelayer B is formed in direct contact with the layer A, and the case whereanother layer (e.g., a layer C or a layer D) is formed in direct contactwith the layer A and the layer B is formed in direct contact with thelayer C or D. Note that another layer (e.g., a layer C or a layer D) maybe a single layer or a plurality of layers.

Similarly, when it is explicitly described that “B is formed above A”,it does not necessarily mean that B is formed in direct contact with A,and another object may be interposed therebetween. Thus, for example,when it is described that “a layer B is formed above a layer A”, itincludes both the case where the layer B is formed in direct contactwith the layer A, and the case where another layer (e.g., a layer C or alayer D) is formed in direct contact with the layer A and the layer B isformed in direct contact with the layer C or D. Note that another layer(e.g., a layer C or a layer D) may be a single layer or a plurality oflayers.

Note that when it is explicitly described that “B is formed in directcontact with A”, it includes not the case where another object isinterposed between A and B but the case where B is formed in directcontact with A.

Note that the same can be said when it is described that B is formedbelow or under A.

Note that when an object is explicitly described in a singular form, theobject is preferably singular. Note that the present invention is notlimited to this, and the object can be plural. Similarly, when an objectis explicitly described in a plural form, the object is preferablyplural. Note that the present invention is not limited to this, and theobject can be singular.

By the present invention, a wide viewing angle display can be realized.Alternatively, a display device which is excellent in contrast can beobtained. Alternatively, a display device higher display quality can beobtained. Alternatively, a display device which is less likely to beinfluenced by noise and can perform clear display can be provided.Alternatively, a display device in which display degradation is lesslikely to be caused can be provided. Alternatively, a display devicewhich has the long life time can be provided.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a diagram showing an example of a pixel structure of thepresent invention;

FIG. 2 is a diagram showing an example of a pixel structure of thepresent invention;

FIG. 3 is a diagram illustrating an example of a display device whichincludes a pixel of the present invention;

FIG. 4 is a diagram illustrating a structural example of a pixelincluded in a display device of the present invention;

FIG. 5 is a diagram showing an example of a pixel structure of thepresent invention;

FIG. 6 is a diagram showing an example of a pixel structure of thepresent invention;

FIG. 7 is a diagram showing an example of a pixel structure of thepresent invention;

FIG. 8 is a diagram showing an example of a pixel structure of thepresent invention;

FIG. 9 is a diagram showing an example of a pixel structure of thepresent invention;

FIG. 10 is a diagram showing an example of a pixel structure of thepresent invention;

FIG. 11 is a diagram showing an example of a pixel structure of thepresent invention;

FIG. 12 is a diagram showing an example of a pixel structure of thepresent invention;

FIG. 13 is a diagram showing an example of a pixel structure of thepresent invention;

FIG. 14 is a diagram showing an example of a pixel structure of thepresent invention;

FIG. 15 is a diagram showing an example of a pixel structure of thepresent invention;

FIG. 16 is a diagram showing an example of a pixel structure of thepresent invention;

FIG. 17 is a diagram illustrating an operation method of a displaydevice shown in FIG. 3;

FIG. 18 is a diagram illustrating an operation of a pixel shown in FIG.5;

FIG. 19 is a diagram showing an example of a pixel structure of thepresent invention;

FIG. 20 a diagram showing an example of a pixel structure of the presentinvention;

FIG. 21 is a diagram showing an example of a pixel structure of thepresent invention;

FIG. 22 is a diagram showing an example of a pixel structure of thepresent invention;

FIG. 23 is a diagram showing an example of a pixel structure of thepresent invention;

FIG. 24 is a diagram showing an example of a pixel structure of thepresent invention;

FIG. 25 is a diagram showing an example of a pixel structure of thepresent invention;

FIG. 26 is a diagram showing an example of a pixel structure of thepresent invention;

FIG. 27 is a diagram showing an example of a pixel structure of thepresent invention;

FIG. 28 is a diagram showing an exemplary structure of a display deviceof the present invention;

FIGS. 29A and 29B are diagrams each showing exemplary structures of adisplay device of the present invention;

FIGS. 30A and 30B are diagrams each showing exemplary structures of adisplay device of the present invention;

FIG. 31 is a diagram showing an exemplary layout of a pixel included ina display device of the present invention;

FIGS. 32A and 32B are diagrams illustrating liquid crystal modes of adisplay device of the present invention;

FIGS. 33A to 33D are diagrams illustrating liquid crystal modes of adisplay device of the present invention;

FIGS. 34A to 34D are diagrams illustrating liquid crystal modes of adisplay device of the present invention;

FIGS. 35A to 35D are diagrams illustrating liquid crystal modes of adisplay device of the present invention;

FIG. 36 is a diagram showing an exemplary structure of a display deviceof the present invention;

FIGS. 37A to 37D are diagrams showing exemplary structures of a displaydevice of the present invention;

FIGS. 38A to 38D are diagrams showing exemplary structures of a displaydevice of the present invention;

FIG. 39 is a diagram showing an example of a peripheral component of adisplay device of the present invention;

FIGS. 40A to 40D are diagrams showing examples of a peripheral componentof a display device of the present invention;

FIGS. 41A and 41B are diagrams showing examples of a peripheralcomponent of a display device of the present invention;

FIG. 42 is a diagram showing an example of a peripheral component of adisplay device of the present invention;

FIGS. 43A to 43C are diagrams showing an exemplary structure of a panelcircuit of a display device of the present invention;

FIGS. 44A to 44C are diagrams showing an exemplary driving method of adisplay device of the present invention;

FIGS. 45A to 45C are diagrams showing an exemplary driving method of adisplay device of the present invention;

FIGS. 46A to 46E are diagrams showing an exemplary driving method of adisplay device of the present invention;

FIGS. 47A and 47B are diagrams showing exemplary driving methods of adisplay device of the present invention;

FIGS. 48A to 48G are diagrams illustrating an exemplary structure oftransistors included in a display device of the present invention;

FIG. 49 is a diagram illustrating an exemplary structure of a transistorincluded in a display device of the present invention;

FIG. 50 is a diagram illustrating an exemplary structure of a transistorincluded in a display device of the present invention;

FIG. 51 is a diagram illustrating an exemplary structure of a transistorincluded in a display device of the present invention;

FIG. 52 is a diagram illustrating an exemplary structure of a transistorincluded in a display device of the present invention;

FIG. 53 is a diagram showing an exemplary structure of a display deviceof the present invention;

FIG. 54 is a diagram showing an exemplary structure of a display deviceof the present invention;

FIGS. 55A and 55B are diagrams showing exemplary structures of a displaydevice of the present invention;

FIGS. 56A and 56B are diagrams showing an exemplary structure of adisplay device of the present invention;

FIG. 57 is a diagram showing an electronic device in which a displaydevice of the present invention is used;

FIG. 58 is a diagram showing an electronic device in which a displaydevice of the present invention is used;

FIGS. 59A to 59H are diagrams showing electronic devices in whichdisplay devices of the present invention are used;

FIG. 60 is a diagram showing an electronic device in which a displaydevice of the present invention is used;

FIG. 61 is a diagram showing an electronic device in which a displaydevice of the present invention is used;

FIG. 62 is a diagram showing an electronic device in which a displaydevice of the present invention is used;

FIG. 63 is a diagram showing an electronic device in which a displaydevice of the present invention is used;

FIGS. 64A and 64B are diagrams showing electronic devices in whichdisplay devices of the present invention are used;

FIGS. 65A and 65B are diagrams showing electronic devices in whichdisplay devices of the present invention are used;

FIGS. 66A to 66C are diagrams showing an exemplary driving method of adisplay device of the present invention;

FIG. 67 is a diagram showing an exemplary driving method of a displaydevice of the present invention;

FIG. 68 is a diagram showing an exemplary driving method of a displaydevice of the present invention;

FIG. 69 is a diagram showing an exemplary driving method of a displaydevice of the present invention;

FIG. 70 is a diagram showing an exemplary driving method of a displaydevice of the present invention;

FIG. 71 is a diagram showing an exemplary driving method of a displaydevice of the present invention;

FIG. 72 is a diagram showing an exemplary driving method of a displaydevice of the present invention;

FIGS. 73A to 73C are diagrams showing exemplary driving methods of adisplay device of the present invention;

FIGS. 74A to 74E are diagrams showing exemplary driving methods of adisplay device of the present invention;

FIGS. 75A and 75B are diagrams showing exemplary driving methods of adisplay device of the present invention;

FIGS. 76A to 76D are diagrams showing an exemplary driving method of adisplay device of the present invention;

FIG. 77 is a diagram showing an example of a pixel structure of thepresent invention;

FIG. 78 is a diagram showing an example of a pixel structure of thepresent invention;

FIG. 79 is a diagram showing an example of a pixel structure of thepresent invention;

FIG. 80 is a diagram showing an exemplary driving method of a displaydevice of the present invention;

FIG. 81 is a diagram showing an example of a pixel structure of thepresent invention;

FIG. 82 is a diagram showing an example of a pixel structure of thepresent invention;

FIG. 83 is a diagram showing an example of a pixel structure of thepresent invention; and

FIG. 84 is a diagram illustrating a conventional technique.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, one mode of the present invention is described. The presentinvention can be carried out in many different modes. It is easilyunderstood by those skilled in the art that modes and details hereindisclosed can be modified in various ways without departing from thepurpose and the scope of the present invention. Accordingly, the presentinvention should not be interpreted as being limited to the embodimentmodes and embodiments. Note that, in a structure of the presentinvention which will be explained below, like reference numerals may beused for like portions throughout the drawings and the description isomitted.

[Embodiment Mode 1]

A basic configuration of a pixel of the present invention is describedwith reference to FIG. 1. A pixel shown in FIG. 1 includes a firstswitch 111, a second switch 112, a third switch 113, a first resistor114, a second resistor 115, a first liquid crystal element 121, a secondliquid crystal element 122, a first storage capacitor 131, and a secondstorage capacitor 132. In addition, the pixel is connected to a signalline 116, a first scan line 117, a second scan line 120, and a Cs line119. Note that each of the first liquid crystal element 121 and thesecond liquid crystal element 122 includes at least a pixel electrode, acommon electrode 118, and liquid crystal controlled by the pixelelectrode and the common electrode 118.

In FIG. 1, the pixel electrode of the first liquid crystal element 121is connected to the signal line 116 through the first switch 111. Inaddition, the pixel electrode of the first liquid crystal element 121 isconnected to the pixel electrode of the second liquid crystal element122 through the second switch 112 and the first resistor 114. When aconnection portion of the first resistor 114 and the pixel electrode ofthe second liquid crystal element 122 is a node 142, the node 142 isconnected to the Cs line 119 through the third switch 113 and the secondresistor 115. In addition, a connection portion of the second switch 112and a wiring which connects the first pixel electrode of liquid crystalelement 121 to the first switch 111 is a node 141.

Note that on/off of the first switch 111 and the second switch 112 iscontrolled by a signal input to the first scan line 117, and on/off ofthe third switch 113 is controlled by both signals input to the firstscan line 117 and the second scan line 120. Although the case where eachswitch is controlled by using scan line is described here, controlmethod of the switch is not limited to this.

An image signal corresponding to a video signal, in other words, apotential based on the gray scale of the pixel is input to the signalline 116.

Although the liquid crystal element shows voltage holding property, theholding rate is not 100%. Therefore, the first storage capacitor 131 andthe second storage capacitor 132 are provided corresponding to the firstliquid crystal element 121 and the second liquid crystal element 122,respectively, in each of the pixels shown in FIG. 1 in order to storethe held voltage. Specifically, the pixel electrode of the first liquidcrystal element 121 is connected to the Cs line 119 through the firststorage capacitor 131, and the pixel electrode of the second liquidcrystal element 122 is connected to the Cs line 119 through the secondstorage capacitor 132. Note that since the voltage holding property ofthe liquid crystal element depends on a liquid crystal material,impurities mixed in the liquid crystal material, the size of a pixel,and the like, storage capacitor need not be provided in the case wherethe voltage holding property of the liquid crystal element is high asshown in FIG. 77. For example, when contribution to display of thesecond liquid crystal element 122 is less than that of the first liquidcrystal element 121, the second storage capacitor 132 which is providedto the second liquid crystal element 122 with less contribution fordisplay is may be omitted.

In addition, in FIG. 1, the node 141 is connected to the node 142through the second switch 112 and the first resistor 114 in this order.Alternatively, the node 141 may be connected to the node 142 through thefirst resistor 114 and the second switch 112 in this order. Further, thenode 142 may be connected to the Cs line 119 through the second resistor115 and the third switch 113 in this order. Of course, as shown in FIG.78, in the second switch 112 and the first resistor 114, and the thirdswitch 113 and the second resistor 115, positions in connection betweenthe switch and the resistor may be reversed.

In addition, each of the first liquid crystal element 121 and the secondliquid crystal element 122 may be formed of a plurality of liquidcrystal elements. Similarly, each of the first storage capacitor 131 andthe second storage capacitor 132 may be formed of a plurality of storagecapacitors. For example, FIG. 79 shows the case where each of the firstliquid crystal element 121 and the second liquid crystal element 122 isformed of two liquid crystal elements and where each of the firststorage capacitor 131 and the second storage capacitor 132 is formed oftwo storage capacitors.

An operation of the pixel in FIG. 1 is described. As described above,on/off of the first switch 111 and the second switch 112 is controlledby inputting a signal to the first scan line 117. Here, the case wherethe first switch 111 and the second switch 112 are turned on byinputting a High level signal (hereinafter, referred to as an H level)to the first scan line 117 is described. In addition, as for the thirdswitch 113 which is controlled by both signals input to the first scanline 117 and the second scan line 120, the case where the third switch113 is turned on only when an H level is input to both first scan line117 and second scan line 120 is described. Thus, in this case, theseswitches are turned off when a Low level signal (hereinafter, referredto as an L level) is input to the first scan line 117, and further, thethird switch 113 is turned off even when an H level is input to thefirst scan line 117 and an L level is input to the second scan line 120.

A period in which the potential based on the gray scale of the pixel isinput to the pixel, in other words, writing period is divided into thefirst half and the latter half by using switches such as the firstswitch 111, the second switch 112, and the third switch 113. The firstswitch 111 and the second switch 112 except for the third switch 113 areturned on in the first half, and the third switch 113 in addition to thefirst switch 111 and the second switch 112 is turned on in the latterhalf. In this manner, the signal line 116 and the Cs line 119 areelectrically disconnected in the first half, and the signal line 116 andthe Cs line 119 are electrically connected in the latter half, whereby avideo signal can be written into a pixel quickly.

First, in the first half of the writing period, an H level is input tothe first scan line 117, and an L level is input to the second scan line120, and then, the first switch 111 and the switch 112 are turned on.The potential which is based on gray scale of a pixel and is input fromthe signal line 116 is supplied to each pixel electrode of the firstliquid crystal element 121 and a first electrode of the first storagecapacitor 131 through the first switch 111. Further, the potential issupplied to the pixel electrode of the second liquid crystal element 122and a first electrode of the second storage capacitor 132 through thesecond switch 112 and the first resistor 114. At that time the thirdswitch 113 is turned off, so that the potential can be supplied to eachpixel electrode of the first liquid crystal element 121 and the secondliquid crystal element 122 quickly.

After that, in the latter half of the writing period, an H level is alsoinput to the second scan line 120, and the third switch 113 in additionto the first switch 111 and the second switch 112 is turned on. In thismanner, the signal line 116 and the Cs line 119 are electricallyconnected. Therefore, the potential which is supplied to each pixelelectrode of the first liquid crystal element 121 and the second liquidcrystal element 122 in the first half of the writing period can beadjusted to the appropriate potential based on the gray scale of thepixel.

Note that the potential which is supplied to the pixel electrode of thesecond liquid crystal element 122 and the first electrode of the secondstorage capacitor 132 is the same as the potential at the node 142, andthe potential is determined by the potential difference between the node141 and the Cs line 119 and the resistance of the first resistor 114 andthe second resistor 115. That is, potential which is based on gray scaleof a pixel and is input from the signal line 116 is divided by the firstresistor 114 and the second resistor 115, and the potential afterresistance division is supplied to the pixel electrode of the secondliquid crystal element 122. Note that when potential which is based ongray scale of a pixel and is input from the signal line 116 is V_(sig),the resistance of the first resistor 114 is R₁, the resistance of thesecond resistor 115 is R₂, and the potential which is supplied to the Csline 119 is V_(cs), the potential of the node 141 is V_(sig), and thepotential of the node 142 in the latter half of the writing period isV_(cs)+(V_(sig)−V_(cs))×R₂/(R₁+R₂).

Note that the potential difference between the potential which is basedon the gray scale and is input from the signal line 116 and thepotential of the Cs line 119 is held in the first storage capacitor 131,and the potential difference between the potential after resistancedivision and the potential of the Cs line 119 is held in the secondstorage capacitor 132.

Next, an L level is input to the first scan line 117. Then, the firstswitch 111, the second switch 112, and the third switch 113 are turnedoff, and the signal line 116, the first liquid crystal element 121, andthe second liquid crystal element 122 are electrically disconnected fromeach other. However, the potential difference between the potentialwhich is based on the gray scale and is input from the signal line 116and the potential of the Cs line 119 is held in the first storagecapacitor 131, and the potential difference between the potential afterresistance division and the potential of the Cs line 119 is kept in thesecond storage capacitor 132. Therefore, the pixel electrode of thefirst liquid crystal element 121 can hold potential which is based ongray scale of a pixel and is input from the signal line 116, and thepixel electrode of second liquid crystal element 122 can hold thepotential after resistance division. Note that an L level may besupplied to the second scan line 120 in addition to the first scan line117 without being limited to the first scan line 117. In any case, thefirst switch 111, the second switch 112, and the third switch 113 may beturned off.

In this manner, the gray scale of the pixel can be expressed using thepotential difference, that is, the voltage held in the first liquidcrystal element 121 and the second liquid crystal element 122. Since thevalue of voltage applied is different between the first liquid crystalelement 121 and the second liquid crystal element 122, liquid crystalwhich is included in each liquid crystal element shows differentorientations. Therefore, viewing angle characteristics can be improved.

Note that since the gray scale that the pixel expresses is determined bythe orientation of the liquid crystal included in each of the firstliquid crystal element 121 and the second liquid crystal element 122 inthe pixel, the potential which corresponds to the gray scale of thepixel and which is supplied from the signal line 116 is necessary to bedetermined in view of these points.

In addition, the signal line 116 and the Cs line 119 are electricallydisconnected in the first half of the writing period, and the signalline 116 and the Cs line 119 are electrically connected in the latterhalf, so that the potential of each pixel electrode of the first liquidcrystal element 121 and the second liquid crystal element 122 can beadjusted to the potential based on the gray scale of the pixel quickly.Thus, a video signal can be written into a pixel quickly.

In addition, the first switch 111, the second switch 112, and the thirdswitch 113 may be controlled by using different scan lines as shown inFIG. 2. FIG. 2 shows the case where the first switch 111 is controlledby a third scan line 201, the case where the second switch 112 iscontrolled by the first scan line 117, and the case where the thirdswitch 113 is controlled by the first scan line 117 and the second scanline 120. Note that the pixel shown in FIG. 2 can be operated in asimilar manner to the pixel shown in FIG. 1.

However, in the case of such a pixel structure, the voltage applied tothe second liquid crystal element 122 is lower than the voltage appliedto the first liquid crystal element 121. Therefore, when the resistanceof the second resistor 115 is much lower than that of the first resistor114, the voltage that is applied to the second liquid crystal element122 is lower than the threshold voltage of liquid crystal, and theliquid crystal included in the second liquid crystal element 122 is notdriven in some cases. Thus, the resistance R₂ of the second resistor 115is preferably higher than the resistance R₁ of the first resistor 114(R₂>R₁). Of course, the relation of the resistances is not limited tothis as long as the liquid crystals of the first liquid crystal element121 and the second liquid crystal element 122 are driven and the grayscale can be expressed using both of the liquid crystals. Note that thethreshold voltage of the liquid crystal indicates a critical value ofthe voltage which is necessary to drive the liquid crystal.

In addition, in FIG. 1, if the resistances of the first resistor 114 andthe second resistor 115 are high and the potential which is input fromthe signal line 116 can be kept at each of the node 141 and the node 142after the first switch 111 is turned off without the second switch 112and the third switch 113, it is not particularly necessary to providethese switches. For example, when the resistance of the first resistor114 is high, the second switch 112 provided between the node 141 and thenode 142 may be omitted, and when the resistance of the second resistor115 is high, the third switch 113 provided between the node 142 and theCs line 119 may be omitted. Of course, when both of the resistances arehigh, the second switch 112 and the third switch 113 can be omitted.

Next, a display device having the above-mentioned pixel shown in FIG. 1is described with reference to FIG. 3.

A display device includes a signal line driver circuit 311, a scan linedriver circuit 312, and a pixel portion 313. The pixel portion 313includes a plurality of signal lines S1 to Sm which are extended fromthe signal line driver circuit 311 in a column direction, first scanlines G1_1 to G1_n, second scan lines G2_1 to G2_n, and Cs lines Cs_1 toCs_n which are extended from the scan line driver circuit 312 in a rowdirection, and a plurality of pixels 314 which are provided in matrixcorresponding to the signal lines S1 to Sm. Then, each pixel 314 isconnected to the signal line Sj (one of the signal lines S1 to Sm), thefirst scan line G1_i (one of the scan lines G1 to Gn), the second scanlines G2_i (one of the scan lines G2 to G2n), and the Cs line Cs_i (oneof the Cs lines Cs_1 to Cs_n).

Note that the signal line Sj, the first scan line G1_i, the second scanline G2_i, and the Cs line Cs_i corresponds to the signal line 116, thefirst scan line 117, the second scan line 120, and the Cs line 119 ofFIG. 1, respectively. The common electrode 118 in FIG. 1 is connected tothe plurality of pixels 314 commonly or electrically, and the samepotential is supplied. Note that when the Cs line 119 and the commonelectrode 118 may have the same potential, the Cs line 119 and thecommon electrode 118 may be electrically connected using a conductivefine particle, wiring, or the like outside the pixel portion 313.

The row of a pixel which is to be operated by inputting a signal intothe first scan line G1_i from the scan line driver circuit 312 isselected sequentially, the potential based on the gray scale of eachpixel is supplied to each pixel that belongs to a selected row from thesignal line driver circuit 311 through the signal lines S1 to Sm.

As shown in FIG. 17, for example, when, i-th row is selected and thewriting period is completed, a signal is written to pixels which belongto i+1-th row. Note that FIG. 17 selectively shows an operation of thefirst switch 111 shown in FIG. 1 which can illustrate a wiring period ineach row precisely. Then, a pixel which is completed the writing periodin i-th row express the gray scale by the voltage held in the firstliquid crystal element and the second liquid crystal element in theperiod.

Note that a signal of an H level is input only to the first scan lineG1_i in the first half of the writing period, and, a signal of an Hlevel is input to second scan line G2_i in the latter half. Thus, thepixel can express the gray scale by the potential supplied from thesignal line Sj in the writing period as described above.

Note that it is desirable to use inversion driving which is driven byinverting the polarity of the voltage to be applied to the pixelelectrode with respect to the potential of the common electrode (commonpotential) in the liquid crystal element every certain period, in orderto suppress liquid crystal material degradation or to decrease displayunevenness such as flickers. In this specification, it is written thatwhen the potential of the pixel electrode is higher than that of thecommon electrode, the voltage of positive polarity is applied to theliquid crystal element, and when the potential of the common electrodeis higher than that of the pixel electrode, the voltage of negativepolarity is applied to the liquid crystal element. In addition, it iswritten that a video signal which is input from the signal line when thevoltage of positive polarity is applied to the liquid crystal element isa signal of positive polarity, and a video signal which is input fromthe signal line when the voltage of negative polarity is applied to theliquid crystal element is a signal of negative polarity. Note thatinversion driving includes, for an example, frame inversion driving,source line inversion driving, gate line inversion driving, dotinversion driving, and the like.

Frame inversion driving is a driving method in which the polarity of thevoltage applied to the liquid crystal element is inverted every oneframe period. Note that one frame period corresponds to a period todisplay an image for one pixel, and there is no particular limitation onone frame period, but one frame period is at least preferably 1/60second or shorter so that a person does not sense flickers.

It is preferable that a period is shortened, and the frequency isincreased further to reduce motion blur. It is desirable that the periodis 1/120 second or shorter (the frequency is 120 Hz or higher). It ismore preferable that the period is 1/180 second or shorter (thefrequency is 180 Hz or higher). When a frame frequency is increased inthis manner, and when the increased frequency does not match the framefrequency of original image data, image data is necessary to beinterpolated. In this case, an image data is interpolated by using amotion vector, so that display at a high frame frequency can berealized. Motion of an image is displayed smoothly, and display withlittle afterimage can be performed in the above described manner.

In addition, source line inversion driving is a driving method in whichthe polarity of the voltage applied to the liquid crystal elements whichbelong to pixels connected to the same signal line is inverted withrespect to the liquid crystal elements which belong to pixels connectedto the adjacent signal line, and further, frame inversion is performedto each pixel. On the other hand, gate line inversion driving is adriving method in which the polarity of the voltage applied to theliquid crystal elements which belong to pixels connected to the samescan line is inverted with respect to the liquid crystal elements whichbelong to pixels connected to the adjacent scan line, and further, frameinversion is performed to each pixel. In addition, dot inversion drivingis a driving method in which the polarity of the voltage applied to theliquid crystal element is inverted between the adjacent pixels, and is adriving method in which source line inversion driving and gate lineinversion driving are combined.

In the meantime, when the frame inversion driving, the source lineinversion driving, the gate line inversion driving, the dot inversiondriving, or the like is employed, the width of the potential which isnecessary for a video signal written in a signal line is twice as largeas the width when the inversion driving is not performed. Therefore, inthe case of the frame inversion driving and the gate line inversiondriving, common inversion driving in which the potential of a commonelectrode is inverted is sometimes employed in order to solve theproblem.

Common inversion driving is a driving method in which the potential of acommon electrode is changed in synchronization with inversion of thepolarity applied to the liquid crystal element, and the width of thepotential which is necessary for a video signal written in a signal linecan be decreased by performing common inversion driving. In this case,it is preferable that the common electrode 118 and the Cs line 119 (theCs line Cs_1 to Cs_n in FIG. 3) are electrically connected. The samesignal is input to the common electrode 118 and the Cs line 119, so thatdisplay is performed more adequately.

For example, when source line inversion driving is performed, positiveand negative video signals in which the potential of the commonelectrode is as a center, in other words, video signals of positivepolarity and negative polarity are supplied alternatively through thesignal line in such a manner that the image signal is changed betweenpositive polarity and negative polarity per frame period. Note that insuch case, a video signal is a signal which can be positive and negativewith respect to the potential supplied to the Cs line.

Next FIG. 4 shows one structural example of a pixel in which dotinversion driving can be realized. FIG. 4 shows four pixels each ofwhich has a structure shown in FIG. 1. In the diagram, signal lines116_1 and 116_2 correspond to the signal line 116 in FIG. 1; first scanlines 117_1 and 117_2 correspond to the first scan line 117; the secondscan lines 120_1 and 120_2 correspond to the second scan line 120; andCs lines 119_1, 119_2, 419_1, and 419_2 correspond to the Cs line 119.Signals having different polarities are input to the signal lines 116_1and 116_2. In accordance with the polarity, potentials which aredifferent from those of the adjacent pixels are supplied using Cs lineswhich are different from those of the adjacent pixels even the pixelsbelong to the same row, that is, by using the Cs lines 119_1 and 419_1,or 119_2 and 419_2 as shown in FIG. 80. Dot inversion driving may beperformed by a driving method as shown in FIG. 80.

Note that when a signal for displaying block color when a normally blackmode is used, and for displaying white color when a normally white modeis used is |V_(sig) (0)|, and the potential of a common electrode isV_(com), a potential which is higher than or equal to V_(com) and lowerthan or equal to V_(sig) (0)+V_(com) may be supplied to the Cs line inthe case where a signal of positive polarity is supplied to the pixelfrom the signal line. On the other hand, a potential which is higherthan or equal to −V_(sig)(0)+V_(com) and lower than or equal to V_(com)may be supplied to the Cs line in the case where a signal of negativepolarity is supplied to the pixel.

Note that it is preferable that a potential which is higher than orequal to V_(com)+α and lower than or equal to V_(sig) (0)+V_(com) besupplied to the Cs line in the case where a signal of positive polarityis supplied to the pixel, and the potential which is higher than orequal to −V_(sig) (0)+V_(com) and lower than or equal to V_(com)−α besupplied to the Cs line in the case where a signal of negative polarityis supplied to the pixel. Here, α is V_(sig) /2. More preferably, apotential of V_(sig) (0)+V_(com) may be supplied in the case where asignal of positive polarity is supplied to the pixel, and a potential of−V_(sig) (0)+V_(com) may be supplied in the case where a signal ofnegative polarity is supplied to the pixel.

As described above, the potential of the Cs line is set at V_(sig)(0)+V_(com) in the case where a signal of positive polarity is suppliedto the pixel, and is set at −V_(sig) (0)+V_(com) in the case where asignal of negative polarity is supplied to the pixel, whereby thevoltage applied to the second liquid crystal element 122 can beincreased, and the second liquid crystal element 122 can be easilycontrolled.

Note that when the potential of the Cs line is set higher than V_(sig)(0)+V_(com) in the case where a signal of positive polarity is suppliedto the pixel, a voltage that is higher than V_(sig) (0) is alwaysapplied to the liquid crystal; therefore, black color cannot bedisplayed in a normally black mode and white color cannot be displayedin a normally white mode. In addition, when the potential which is lowerthan −V_(sig) (0)+V_(com) is supplied to the Cs line in the case where asignal of negative polarity is supplied to the pixel, black color cannotbe displayed in a normally black mode and white color cannot bedisplayed in a normally white mode, similar to the case of the signal ofpositive polarity.

Note that FIG. 80 shows a case where the potential of V_(com) is 0 V,the potential of the Cs line in the case where a signal of positivepolarity is supplied to the pixel is V_(sig) (0), and the potential ofthe Cs line in the case where a signal of negative polarity is suppliedto the pixel is −V_(sig) (0).

Note that the method of inversion driving is not limited to the abovedescribed methods.

In addition, a wiring which is connected to each pixel is shared betweenpixels, whereby the number of the wiring can be reduced. In this case,various wiring can be shared between pixels as long as it operatesnormally. For example, a wiring can be shared with the pixel of the nextrow, and an example is described.

A pixel 500 shown in FIG. 5 includes the first switch 111, the secondswitch 112, the third switch 113, the first resistor 114, the secondresistor 115, the first liquid crystal element 121, the second liquidcrystal element 122, the first storage capacitor 131, and the secondstorage capacitor 132 similar to FIG. 1. Note that the pixel 500 isconnected to the signal line 116, a first scan line 517, the Cs line119, and the first scan line 517 of the next row.

The first scan line 517 of the next row is used as one of scan lineswhich control the third switch 113 in the pixel shown in FIG. 5, whereasthe second scan line 120 is used in FIG. 1. As described above bysharing wiring with next row, the number of the wirings can be reducedto improve the aperture ratio.

Note that when the third switch 113 is turned on, the first switch 111and the second switch 112 in a pixel of the next row are turned onbefore writing is completed in the pixel shown in FIG. 5. That is, apixel of the next row is selected in a latter half of the writing periodin the pixel 500 as shown in FIG. 18. Note that FIG. 18 shows thewriting period in i−1-th row, i-th row, i+1-th row and selectivelyillustrates an operation of the first switch 111 which can illustrate awiring period precisely similar to FIG. 17.

As described above, the voltage which is applied to the first liquidcrystal element 121 and the second liquid crystal element 122 includedin the pixel of the next row changes from the voltage based on the grayscale of the pixel because the pixel of the next row is selected in thelatter half of the writing period. However, since a video signal iswritten to the pixel of the next row after the pixel 500, a period whenthe third switch 113 is turned on, in other words, the latter half ofthe writing period is set so that the display is not influenced, wherebychanging the voltage does not cause a problem in particular. Needless tosay, in a pixel structure of FIG. 2, in other words, in the case where ascan line which controls the first switch 111 is provided separatelyfrom the first scan line 517, such a thing does not occur.

Note that in the foregoing case, various forms of switches can be usedfor the first switch 111, the second switch 112, and the third switch113, and an electric switch or a mechanical switch can be applied to theswitches. That is, any element can be used as long as it can control acurrent flow, without limitation to a particular element. For example,the switch may be a transistor, a diode, or a logic circuit withcombines them.

As shown in FIG. 6, a structure in which a second transistor 612 and athird transistor 613 are used for the second switch 112 and the thirdswitch 113 of FIG. 1 respectively, and further, the first resistor 114and the second resistor 115 of FIG. 1 are realized by usingon-resistance of these transistors may be employed to omit theseresistances. Note that the third transistor 613 is necessary to becontrolled by signals input to the first scan line 117 and the secondscan line 120. Therefore, the third transistor 613 is configured by twotransistors 620 and 621 of which the gate electrode is connected to thefirst scan line 117 and the second scan line 120, respectively.

Note that as described above, since the resistance R₂ of the secondresistor 115 is preferably higher than the resistance R₁ of the firstresistor 114 (R₂>R₁) in FIG. 1, the on-resistance of the thirdtransistor 613 is preferably higher than the on-resistance of the secondtransistor 612 also in the structure of FIG. 6. Therefore, when thechannel width and the channel length of the second transistor 612 are W₂and L₂, respectively, and the channel width and the channel length ofthe third transistor 613 are W₃ and L₃, respectively, transistors whichsatisfy the relation, W₂/L₂>W₃/L₃, are preferably used. Here, whenchannel widths W of the serially-connected transistors 620 and 621 areequal, the value of channel length L of the third transistor 613corresponds to a total channel length of each transistor. However, thepresent invention is not limited to this relation. Needless to say thepresent invention is not limited to this pixel structure, and as shownin FIG. 81 for example, a structure in which the second transistor 612and the third transistor 613 are used for the second switch 112 and thethird switch 113 of FIG. 1 respectively, and resistors are not omittedmay be employed.

In addition, when a transistor (here, referred to as a first transistor)is used for the first switch 111 in FIG. 1, the on-resistance of thefirst transistor is preferably lower. When the channel width and thechannel length of the first transistor are W₁ and L₁, respectively,W₁/L₁ is preferably larger. FIG. 83 shows the case where the firsttransistor 8411 is used for the first switch 111 in the structure ofFIG. 6. Note that transistors which, satisfy the relation,W₁/L₁>W₂/L₂>W₃/L₃, when the first transistor 8411 is compared to thesecond transistor 612 and the third transistor 613, are preferably used.However, the present invention is not limited to this.

In addition, as for relation of connection of the transistors 620 and621 which configures the third transistor 613, the node 142 may beprovided so as to be connected to the Cs line 119 through the transistor620 and the transistor 621 in this order as shown in FIG. 6, and thenode 142 may be provided so as to be connected to the Cs line 119through the transistor 621 and the transistor 620 in this order as shownin FIG. 7. In a pixel structure shown in FIG. 7, the transistor which isconnected to the node 142 among transistors which configures the thirdtransistor 613 is off, whereby the gate capacitance of the transistorcan be smaller than the case that the gate capacitance of transistor ison. Therefore, the potential can be supplied to each pixel electrode ofthe first liquid crystal element 121 and the second liquid crystalelement 122 quickly in the first half of the writing period compared toFIG. 6.

In addition, a multi-gate transistor 821 in which two transistors areconnected in series may be used for a transistor 621 which configuresthe third transistor 613 of FIG. 6 in order to make the on-resistance ofthe third transistor 613 larger as shown in FIG. 8. Note that althoughFIG. 8 illustrates the case where two transistors are connected inseries, the number of serially connected transistors is not limited inparticular.

Note that when the channel widths W of two transistors which isconnected in series are equal, the channel length L of the transistor821 is the total channel length of the two transistors. Therefore, W/Leasily becomes smaller, and the on-resistance can be increased. Thus,the on-resistance of the transistor 821 can be easily made higher byusing the multi-gate transistor. Thus, the on-resistance of the thirdtransistor 613 can be easily made higher than that of the secondtransistor 212.

In addition, a multi-gate transistor 920 may be used for the transistor620 in FIG. 6 as shown in FIG. 9 without limiting to the transistor 621.

In addition, when the voltage applied to the second liquid crystalelement 122 is lower than the threshold voltage of the liquid crystalincluded in the second liquid crystal element 122, for example in thecase the on-resistance of the third transistor 613 is much lower thanthat of the second transistor 612, a diode-connected transistor 1014which is serially connected to third transistor 613 may be provided as aresistor as shown in FIG. 10.

By the diode-connected transistor 1014, the voltage which is at leastequal to or higher than the threshold voltage of the transistor 1014 canbe held in the second storage capacitor 132. Therefore, by using thediode-connected transistor 1014, the voltage applied to the secondliquid crystal element 122 can be increased, and the liquid crystalincluded in the second liquid crystal element 122 can be driven moresurely. Since diodes have non-linearity, the resistance becomes largerin a region where the voltage is low; accordingly, diodes areparticularly effective in such a case. It is needless to say that aresistor can also be used. Here, the drawings are illustrated on theassumption that the potential which is based on the gray scale and isinput from the signal line 116 is positive, and an example in which ann-channel transistor is used for the transistor 1014 and a drainelectrode of the transistor 1014 is connected to the third transistor613 is shown. Of course, a p-channel transistor can be used for thetransistor 1014. However, a source electrode is connected to the thirdtransistor 613 in this case.

Note that when the source line inversion driving, the dot inversiondriving or the like is performed as described above, positive andnegative image signals with the potential of the common electrode as acenter, in other words, image signals of positive polarity and negativepolarity are supplied alternatively through a signal line in such amanner that the image signal is changed between positive polarity andnegative polarity per frame period. In such case, an image signal is asignal which can be positive and negative with respect to the potentialsupplied to the Cs line. Therefore, the potential of the Cs line 119 maybe changed between the time an image signal of positive polarity isinput and the time an image signal of negative polarity is input. Inother words, the potential of the Cs line 119 when a signal of negativepolarity is input may be lower than that when a signal of positivepolarity is input. Thus, the voltage can be supplied in each pixelelectrode adequately. Note that the pixel shown in FIG. 10 may employ astructure shown in FIG. 11 in order to deal with both a positive imagesignal and a negative image signal. FIG. 11 shows a structure of a pixelin which a diode-connected transistor 1114 which is connected to thediode-connected transistor 1014 in FIG. 10 in parallel is furtherprovided. Note that when these transistors are transistors having thesame conductivity type, the transistor 1014 and the transistor 1114 areconnected so that directions of the current flow are different from eachother. By this structure, even if relation between the potential of Csline 119 and the potential supplied from the signal line 116 isreversed, the voltage which is at least equal to or higher than thethreshold voltage of the transistor 1014 or the transistor 1114 can beheld in the second storage capacitor 132. Therefore, the voltage appliedto the second liquid crystal element 122 can be increased, and theliquid crystal included in the second liquid crystal element 122 can bedriven more surely. Note that the pixel structure shown in FIG. 11 maybe used even when such a driving method is not used.

In addition, different potentials may be supplied to the commonelectrode 118 and the Cs line 119, or the same potential may be suppliedthereto in this specification. In addition, as shown in FIG. 82, the Csline 119 may be combined with the common electrode 118. Note that FIG.82 shows a case where a wiring 8300 is used as a combination of thecommon electrode 118 and Cs line 119 of FIG. 1.

FIGS. 1 to 11 show the case where two liquid crystal elements areprovided in one pixel; however the number of the liquid crystal elementsincluded in a pixel is not limited in particular. FIG. 12 shows the casewhere three liquid crystal elements are provided in one pixel. A pixelshown in FIG. 12 includes a transistor 1214, a liquid crystal element1223 and a storage capacitor 1233 in addition to the pixel structureshown in FIG. 6. In FIG. 12, a pixel electrode of the liquid crystalelement 1223 is connected to the signal line 116 through the secondtransistor 612 and the first switch 111. When a connection portion ofthe pixel electrode of the liquid crystal element 1223 and the secondtransistor 612 is a node 1200, the node 1200 is connected to the node142 through the transistor 1214. Note that a gate electrode of thetransistor 1214 is connected to the first scan line 117 similar to thesecond transistor 612 and the third transistor 613. In addition, thenode 1200 is connected to the Cs line 119 through the third storagecapacitor 1233. In this manner, a unit 1201 which includes thetransistor 1214, the liquid crystal element 1223, and the storagecapacitor 1233 is provided between the second transistor 612 and thenode 142 in FIG. 12. Note that when the number of the liquid crystalelements included in a pixel is increased, for example, the number ofunits 1201 may be increased. Of course, the present invention is notlimited to this.

In addition, a plurality of pixel structures which are mentioned abovemay be provided in one pixel. FIG. 13 shows one structural example ofsuch a pixel. A pixel shown in FIG. 13 includes two sub-pixels 1300 aand 1300 b, and gray scale of one pixel is expressed using thesesub-pixels. In FIG. 13, the pixel structure shown in FIG. 6 is employedfor each of the sub-pixels. Note that the signal line 116, the firstscan line 117, and the second scan line 120 which are connected to thesub-pixels 1300 a and 1300 b are shared by the sub-pixels. The differentpotentials can be supplied to each Cs line 119 which is connected tosub-pixels 1300 a and 1300 b, so that different voltages can be appliedto the liquid crystal elements which belong to different sub-pixels. Inthis way, the viewing angle characteristics can be further improvedusing a difference of the orientation of the liquid crystal in thesub-pixels.

In addition, the case where the signal line 116, the first scan line117, and the second scan line 120 are utilized as a common wiring in thesub-pixels as shown in FIG. 13 has been described; however, only thefirst scan line 117 and the second scan line 120 may be shared bysub-pixels 1400 a and 1400 b as shown in FIG. 14. In addition, only thesignal line 116 may be shared by sub-pixels 1500 a and 1500 b as shownin FIG. 15, and the gray scale of one pixel may be expressed using thesesub-pixels. Note that the wiring which is shared by the sub-pixels isnot limited to the above line, and the Cs line 119 may be used, or twoor more wirings may be shared as shown in FIGS. 13 and 14.

The shared wiring is not necessarily a wiring which has similar functionamong sub-pixels. For example, as shown in FIG. 16, a scan line which isdifferent from the first scan line 117 among scan lines which controlsthe transistor 613 included in one sub-pixel 1600 a, that is, the firstscan line 117 included in the other sub-pixel 1600 b which is located onthe next portion, can be used.

Note that in the pixel shown in FIG. 16, the first switch 111 and thetransistor 612 included in a pixel of the next row is turned on beforethe writing for the pixel which includes the sub-pixel 1600 a and thesub-pixel 1600 b is completed. Therefore, the voltage which is appliedto the first liquid crystal element 121 and the second liquid crystalelement 122 included in the pixel of the next row changes from thevoltage based on the gray scale of the pixel. However, similar to thepixel structure of FIG. 6, since a video signal is written to the pixelof the next row after the pixel which includes the sub-pixel 1600 a andsub-pixel 1600 b, a period when the transistor 613 is turn on, in otherwords the latter half of the writing period is set so that the displayis not influenced, whereby changing the voltage does not cause a problemin particular.

In addition, FIGS. 13 to 16 show the case where one pixel includessub-pixels having the same structure; however, the structures may bedifferent among sub-pixels. In addition, the case where the first switch111, the transistor 612 and the transistor 613 are controlled by usingthe same scan line is described mainly; alternatively, different scanlines may be used for controlling switches as shown in FIG. 2.

In the above-described manner, the viewing angle characteristics can beimproved by the present invention. Further, the present invention can bedriven without reduction in contrast; therefore, a liquid crystaldisplay device which has higher display quality can be provided.

Although this embodiment mode is described with reference to variousdrawings, the contents (or a part thereof) described in each drawing canbe freely applied to, combined with, or replaced with the contents (or apart thereof) described in another drawing. Further, even more drawingscan be formed by combining each part with another part in theabove-described drawings.

Similarly, the contents (or a part thereof) shown in each drawing ofthis embodiment mode can be freely applied to, combined with, orreplaced with the contents (or a part thereof) described in a drawing inanother embodiment mode. Further, even more drawings can be formed bycombining each part with part of another embodiment mode in the drawingsof this embodiment mode.

Note that this embodiment mode shows an example of an embodied case ofthe contents (or a part thereof) described in other embodiment modes, anexample of slight transformation thereof, an example of partialmodification thereof, an example of improvement thereof, an example ofdetailed description thereof, an application example thereof, an exampleof related part thereof, or the like. Therefore, the contents describedin other embodiment modes can be freely applied to, combined with, orreplaced with this embodiment mode.

[Embodiment Mode 2]

In this embodiment mode, an example of the pixel structure which isdifferent from that of Embodiment Mode 1 is described. A pixel shown inFIG. 19 includes the first switch 111, a second switch 1712, the thirdswitch 113, the first resistor 114, the second resistor 115, the firstliquid crystal element 121, the second liquid crystal element 122, thefirst storage capacitor 131, and the second storage capacitor 132. Inaddition, the pixel is connected to the signal line 116, the first scanline 117, the second scan line 120, and the Cs line 119.

Note that on/off of the first switch 111 is controlled by a signal inputto the first scan line 117, and on/off of the second switch 1712 and thethird switch 113 is controlled by both signals input to the first scanline 117 and the second scan line 120. As described above, the pixelstructure shown in FIG. 19 is different from the pixel structure shownin FIG. 1 in that the second switch 1712 is controlled by both signalsof the first scan line 117 and the second scan line 120. Referencenumerals denoting the same components as those in FIG. 1 are used incommon throughout the drawings, and the description is omitted.

In a pixel shown in FIG. 19, a writing period is divided into the firsthalf and the latter half by using the first switch 111, the secondswitch 1712, and the third switch 113 in the similar manner to the pixelshown in FIG. 1

Note that the case is described where the first switch 111 is turned onwhen an H level is input to the first scan line 117, and the secondswitch 1712 and the third switch 113 are turned on only when an H levelis input to both of the first scan line 117 and the second scan line120.

First, in the first half of the writing period, an H level is input tothe first scan line 117, and an L level is input to the second scan line120, and then, the first switch 111 is turned on. At that time thesecond switch 1712 and the third switch 113 are turned off, so that thepotential can be supplied to the pixel electrode of the first liquidcrystal element 121 quickly.

After that, in the latter half of the writing period, an H level is alsoinput to the second scan line 120, and then, the second switch 1712 andthe third switch 113 as well as the first switch 111 can be turned on.In this manner, the signal line 116 and the Cs line 119 which areelectrically disconnected are electrically connected. Therefore, thepotential which is supplied to the pixel electrode of the first liquidcrystal element 121 in the first half of the writing period can beadjusted to the appropriate potential based on the gray scale of thepixel quickly. In addition, the potential based on the gray scale of thepixel is supplied to the pixel electrode of the second liquid crystalelement 122.

In this manner, the gray scale of the pixel can be expressed using thepotential difference, that is, the voltage, held in the first liquidcrystal element 121 and the second liquid crystal element 122. Since thevalue of voltage applied is different between the first liquid crystalelement 121 and the second liquid crystal element 122, liquid crystalwhich is included in each liquid crystal element shows differentorientations. Therefore, viewing angle characteristics can be improved.In addition, a video signal can be written into the pixel quickly.

Note that the gray scale that the pixel expresses is determined by theorientation of the liquid crystal which is included in each of the firstliquid crystal element 121 and the second liquid crystal element 122 inthe pixel, whereby the potential supplied from the signal line 116should be determined in consideration of these factors.

In addition various forms of switches can be used for the first switch111, the second switch 1712, and the third switch 113, and an electricswitch or a mechanical switch can be applied to the switches. That is,any element can be used as long as it can control a current flow,without limitation to a particular element. For example, the switch maybe a transistor, a diode, or a logic circuit with combines them.

Therefore, similar to FIG. 6, a structure in which a second transistor1722 and a third transistor 613 are used for the second switch 1712 andthe third switch 113 of FIG. 19, respectively, as shown in FIG. 20, andfurther, the first resistor 114 and the second resistor 115 of FIG. 19may be realized by using on-resistance of these transistors may beemployed to omit these resistances. Note that the second transistor 1722and the third transistor 613 are needed to be controlled by both signalsinput to the first scan line 117 and the second scan line 120.Therefore, each of the second transistor 1722 and the third transistor613 is configured by two transistors of which gate electrode isconnected to the first scan line 117 and the second scan line,respectively.

In addition, in order to electrically disconnect the signal line 116 andthe Cs line 119 in the first half of the writing period, only the thirdswitch 113 are used in FIG. 1 in Embodiment Mode 1, and the secondswitch 1712 and the third switch 113 is used in FIG. 19; however, thestructure is not limited to these structure. For example, only thesecond switch 1712 may be used as shown in FIG. 21. In this case, on/offof the first switch 111 and the third switch 1733 are controlled by asignal input to the first scan line 117, and on/off of the second switch1712 is controlled by both signals input to the first scan line 117 andthe second scan line 120.

In addition, the second transistor 1722 and a third transistor 1743 areused for the second switch 1712 and the third switch 1733 in FIG. 21,respectively, as shown in FIG. 22, and further, the first resistor 114and the second resistor 115 in FIG. 21 may be realized by usingon-resistance of these transistors to omit these resistances.

In addition, the number of liquid crystal elements included in one pixelis not limited in particular as described in Embodiment Mode 1. Forexample, a unit which includes a transistor 1750, a liquid crystalelement 1751, and a storage capacitor 1752 may be further providedbetween the node 141 and the transistor 1722 in FIG. 20 as shown in FIG.23. Note that in the first half of the writing period, a switch forelectrically disconnecting the signal line 116 and the Cs line 119 isnot limited to the transistor 1722 and the transistor 613, and thesignal line 116 and the Cs line 119 are electrically disconnected byusing a transistor included in the unit.

In the above-described manner, the viewing angle characteristics can beimproved by the present invention. Further, the present invention can bedriven without reduction in contrast; therefore, a liquid crystaldisplay device which has higher display quality can be provided.

Although this embodiment mode is described with reference to variousdrawings, the contents (or a part thereof) described in each drawing canbe freely applied to, combined with, or replaced with the contents (or apart thereof) described in another drawing. Further, even more drawingscan be formed by combining each part with another part in theabove-described drawings.

Similarly, the contents (or a part thereof) shown in each drawing ofthis embodiment mode can be freely applied to, combined with, orreplaced with the contents (or a part thereof) described in a drawing inanother embodiment mode. Further, even more drawings can be formed bycombining each part with part of another embodiment mode in the drawingsof this embodiment mode.

Note that this embodiment mode shows an example of an embodied case ofthe contents (or a part thereof) described in other embodiment modes, anexample of slight transformation thereof, an example of partialmodification thereof, an example of improvement thereof, an example ofdetailed description thereof, an application example thereof, an exampleof related part thereof, or the like. Therefore, the contents describedin other embodiment modes can be freely applied to, combined with, orreplaced with this embodiment mode.

[Embodiment Mode 3]

In this embodiment mode, an example of the pixel structure which isdifferent from that of Embodiment Mode 1. A pixel shown in FIG. 24includes the switch 111, the transistor 612, the transistor 613, thefirst liquid crystal element 121, the second liquid crystal element 122,the first storage capacitor 131, the second storage capacitor 132, and athird storage capacitor 1901. Note that the pixel shown in FIG. 24 isconnected to the signal line 116, the first scan line 117, the secondscan line 120, and the Cs line 119, and has a structure in which thethird storage capacitor 1901 is provided between the node 142 and oneelectrode of the transistor 620 which is one transistor that configuresthe transistor 613 in the pixel shown in FIG. 6 in Embodiment Mode 1.Note that reference numerals denoting the same components as those inFIG. 6 are used in common throughout the drawings, and the descriptionis omitted.

The pixel shown in FIG. 24 can be operated in a similar manner to thepixel shown in FIG. 6. Note that by providing the third storagecapacitor 1901, it takes time to obtain the potential which correspondsto the gray scale that should be supplied to the pixel electrode of thesecond liquid crystal element 122. Therefore, response speed of theliquid crystal included in the second liquid crystal element 122 towhich lower voltage is applied compared to the first liquid crystalelement 121 is intentionally made slower, whereby viewing anglecharacteristics can be improved. In this case also, the gray scale thatthe pixel expresses is determined by the orientation of the liquidcrystal which is included in each of the first liquid crystal element121 and the second liquid crystal element 122 in the pixel, whereby thepotential supplied from the signal line 116 is determined inconsideration of these factors.

In addition, the third storage capacitor 1911 may be provided betweenthe transistor 612 and the node 142 as shown in FIG. 25. The pixel shownin FIG. 25 can be operated in a similar manner to the pixel shown inFIG. 6. It takes time to obtain the potential which corresponds to thegray scale that should be supplied to the pixel electrode of the secondliquid crystal element 122, similar to the pixel shown in FIG. 24.Viewing angle characteristics can be further improved by using this. Inthis case, the potential supplied from the signal line 116 needed to bedetermined by taking into account the factor that the voltage applied tothe second liquid crystal element 122 is determined by capacitancedivision with the third storage capacitor 1911.

Also in the above mentioned pixel structure, the gray scale that thepixel expresses is determined by the orientation of the liquid crystalwhich is included in each of the first liquid crystal element 121 andthe second liquid crystal element 122 in the pixel in a similar mannerto Embodiment Mode 1, whereby the viewing angle characteristics can beimproved. Further, the present invention can be driven without reductionin contrast; therefore, a liquid crystal display device which has higherdisplay quality can be provided.

Although this embodiment mode is described with reference to variousdrawings, the contents (or a part thereof) described in each drawing canbe freely applied to, combined with, or replaced with the contents (or apart thereof) described in another drawing. Further, even more drawingscan be formed by combining each part with another part in theabove-described drawings.

Similarly, the contents (or a part thereof) shown in each drawing ofthis embodiment mode can be freely applied to, combined with, orreplaced with the contents (or a part thereof) described in a drawing inanother embodiment mode. Further, even more drawings can be formed bycombining each part with part of another embodiment mode in the drawingsof this embodiment mode.

Note that this embodiment mode shows an example of an embodied case ofthe contents (or a part thereof) described in other embodiment modes, anexample of slight transformation thereof, an example of partialmodification thereof, an example of improvement thereof, an example ofdetailed description thereof, an application example thereof, an exampleof related part thereof, or the like. Therefore, the contents describedin other embodiment modes can be freely applied to, combined with, orreplaced with this embodiment mode.

[Embodiment Mode 4]

In this embodiment mode, an example of the pixel structure which isdifferent from those of Embodiment Modes 1 to 3 is described. The pixelshown in FIG. 26 includes the switch 111, the transistor 612, thetransistor 613, a transistor 1924, the first liquid crystal element 121,the second liquid crystal element 122, the first storage capacitor 131,the second storage capacitor 132, and a third storage capacitor 1921.Note that the pixel shown in FIG. 26 is connected to the signal line116, the first scan line 117, the second scan line 120, and the Cs line119, and has a structure where the transistor 1924 and the third storagecapacitor 1921 are further provided in the pixel shown in FIG. 6 inEmbodiment Mode 1. Reference numerals denoting the same components asthose in FIG. 6 are used in common throughout the drawings, and thedescription is omitted.

When a connection portion of the node 142 and the first electrode of thesecond storage capacitor 132 is a node 1922, the third storage capacitor1921 is provided between the node 1922 and the pixel electrode of thesecond liquid crystal element 122. In addition, when a connectionportion of the third storage capacitor 1921 and the pixel electrode ofthe second liquid crystal element 122 is a node 1923, the transistor1924 is provided between the node 1923 and the signal line 116. In otherwords, the node 1923 is connected to the signal line 116 through thetransistor 1924. On/off of the transistor 1924 is controlled by a signalinput to the first scan line 117 as in the case of the switch 111, thetransistor 612, and the transistor 620.

The pixel shown in FIG. 26 can be operated in a similar manner to thepixel shown in FIG. 6. Note that the potential is supplied to the pixelelectrodes of the first liquid crystal element 121 and the second liquidcrystal element 122 from the signal line 116 through the switch 111 andthe transistor 1924, respectively, at the same time. Therefore, thepotential of the pixel electrode of each liquid crystal element can beadjusted to the appropriate potential based on the gray scale of thepixel quickly. Thus, it is effective in the case of high speedoperation. In addition, also in the pixel shown in FIG. 26, the grayscale that the pixel expresses is determined by the orientation of theliquid crystal which is included in each of the first liquid crystalelement 121 and the second liquid crystal element 122 in the pixel,whereby the viewing angle characteristics can be improved. Note that itis preferable that the on-resistance of the transistor 1924 be higherthan that of the switch 111.

Thus, viewing angle characteristics can be improved by the presentinvention. Further, the present invention can be driven withoutreduction in contrast; therefore, a liquid crystal display device whichhas higher display quality can be provided.

Although this embodiment mode is described with reference to variousdrawings, the contents (or a part thereof) described in each drawing canbe freely applied to, combined with, or replaced with the contents (or apart thereof) described in another drawing. Further, even more drawingscan be formed by combining each part with another part in theabove-described drawings.

Similarly, the contents (or a part thereof) shown in each drawing ofthis embodiment mode can be freely applied to, combined with, orreplaced with the contents (or a part thereof) described in a drawing inanother embodiment mode. Further, even more drawings can be formed bycombining each part with part of another embodiment mode in the drawingsof this embodiment mode.

Note that this embodiment mode shows an example of an embodied case ofthe contents (or a part thereof) described in other embodiment modes, anexample of slight transformation thereof, an example of partialmodification thereof, an example of improvement thereof, an example ofdetailed description thereof, an application example thereof, an exampleof related part thereof, or the like. Therefore, the contents describedin other embodiment modes can be freely applied to, combined with, orreplaced with this embodiment mode.

[Embodiment Mode 5]

In this embodiment mode, an example of the pixel structure which isdifferent from those of Embodiment Modes 1 to 4 is described. The pixelshown in FIG. 27 includes the switch 111, the transistor 612, thetransistor 613, the first liquid crystal element 121, the second liquidcrystal element 122, the first storage capacitor 131, the second storagecapacitor 132, and a third storage capacitor 1931. Note that the pixelshown in FIG. 27 is connected to the signal line 116, the first scanline 117, the second scan line 120, and the Cs line 119, and has astructure where the third storage capacitor 1931 is further provided inthe pixel shown in FIG. 6 in Embodiment Mode 1. Reference numeralsdenoting the same components as those in FIG. 6 are used in commonthroughout the drawings, and the description is omitted.

When a connection portion of the node 142 and the first electrode of thesecond storage capacitor 132 is a node 1932, the third storage capacitor1931 is provided between the node 1932 and the pixel electrode of thesecond liquid crystal element 122.

The pixel shown in FIG. 27 can be operated in a similar manner to thepixel shown in FIG. 6. Note that by providing the third storagecapacitor 1931, it takes time to obtain the potential which correspondsto the gray scale that should be supplied to the pixel electrode of thesecond liquid crystal element 122. Viewing angle characteristics can befurther improved by using this. In addition, since the voltage appliedto the second liquid crystal element 122 is determined by capacitancedivision with the third storage capacitor 1931, providing the thirdstorage capacitor 1931 is effective when low voltage designed to beapplied to the second liquid crystal element 122.

Also in the pixel of this embodiment mode, the gray scale that the pixelexpresses is determined by the orientation of the liquid crystal whichis included in each of the first liquid crystal element 121 and thesecond liquid crystal element 122 in the pixel, whereby the viewingangle characteristics can be improved.

Although this embodiment mode is described with reference to variousdrawings, the contents (or a part thereof) described in each drawing canbe freely applied to, combined with, or replaced with the contents (or apart thereof) described in another drawing. Further, even more drawingscan be formed by combining each part with another part in theabove-described drawings.

Similarly, the contents (or a part thereof) shown in each drawing ofthis embodiment mode can be freely applied to, combined with, orreplaced with the contents (or a part thereof) described in a drawing inanother embodiment mode. Further, even more drawings can be formed bycombining each part with part of another embodiment mode in the drawingsof this embodiment mode.

Note that this embodiment mode shows an example of an embodied case ofthe contents (or a part thereof) described in other embodiment modes, anexample of slight transformation thereof, an example of partialmodification thereof, an example of improvement thereof, an example ofdetailed description thereof, an application example thereof, an exampleof related part thereof, or the like. Therefore, the contents describedin other embodiment modes can be freely applied to, combined with, orreplaced with this embodiment mode.

[Embodiment Mode 6]

In this embodiment mode, a pixel structure of a display device isdescribed. In particular, a pixel structure of a liquid crystal displaydevice is described.

A pixel structure in the case where each liquid crystal mode and atransistor are combined is described with reference to cross-sectionalviews of a pixel.

Note that as the transistor, a thin film transistor (a TFT) or the likeincluding a non-single crystalline semiconductor layer typified byamorphous silicon, polycrystalline silicon, micro crystalline (alsoreferred to as semi-amorphous) silicon, or the like can be used. As astructure of the transistor, a top-gate structure, a bottom-gatestructure, or the like can be used. Note that a channel-etchedtransistor, a channel-protective transistor, or the like can be used asa bottom-gate transistor.

FIG. 28 is an example of a cross-sectional view of a pixel in the casewhere a TN mode and a transistor are combined. Features of the pixelstructure shown in FIG. 28 are described.

Liquid crystal molecules 2018 shown in FIG. 28 are long and narrowmolecules each having a major axis and a minor axis. In FIG. 28, adirection of each of the liquid crystal molecules 2018 is expressed bythe length thereof. That is, the direction of the major axis of theliquid crystal molecule 2018, which is expressed as long, is parallel tothe page (a cross-sectional direction shown in FIG. 28), and as theliquid crystal molecule 2018 is expressed to be shorter, the directionof the major axis becomes closer to a normal direction of the page. Thatis, among the liquid crystal molecules 2018 shown in FIG. 28, thedirection of the major axis of the liquid crystal molecule 2018 which isclose to the first substrate 2001 and the direction of the major axis ofthe liquid crystal molecule 2018 which is close to the second substrate2016 are different from each other by 90 degrees, and the directions ofthe major axes of the liquid crystal molecules 2018 located therebetweenare arranged so as to link the above two directions smoothly. That is,the liquid crystal molecules 2018 shown in FIG. 28 are aligned to betwisted by 90 degrees between the first substrate 2001 and the secondsubstrate 2016.

Note that the case is described in which a bottom-gate transistor usingan amorphous semiconductor is used as the transistor. In the case wherea transistor using an amorphous semiconductor is used, a liquid crystaldisplay device can be formed at low cost by using a large substrate.

A liquid crystal display device includes a basic portion displayingimages, which is called a liquid crystal panel. The liquid crystal panelis manufactured as follows: two processed substrates are attached toeach other with a gap of several μm therebetween, and a liquid crystalmaterial is injected into a space between the two substrates. That is,the liquid crystal is interposed between the first substrate and thesecond substrate. In FIG. 28, liquid crystal 2011 is interposed betweenthe first substrate 2001 and the second substrate 2016. A transistor anda pixel electrode are foil led over the first substrate. Alight-shielding film 2014, a color filter 2015, a fourth conductivelayer 2013, a spacer 2017, and a second alignment film 2012 are formedover the second substrate 2016.

When the light-shielding film 2014 is formed, a display device withlittle light leakage at the time of black display can be obtained. Thelight-shielding film 2014 is not necessarily provided over the secondsubstrate 2016. When the light-shielding film 2014 is not formed, thenumber of steps can be reduced, so that manufacturing cost can bereduced and yield can be improved.

The color filter 2015 is not necessarily provided over the secondsubstrate 2016. When the color filter 2015 is not formed, the number ofsteps can be reduced similarly to when the light-shielding film is notformed, so that manufacturing cost can be reduced and yield can beimproved. Note that even when the color filter 2015 is not formed, adisplay device which can perform color display can be obtained by fieldsequential driving.

Spherical spacers may be dispersed over the second substrate 2016instead of forming the spacer 2017. When the spherical spacers aredispersed, the number of steps is reduced, so that manufacturing costcan be reduced. In addition, yield can be improved. Alternatively, whenthe spacer 2017 is formed, a distance between the two substrates can beuniform easily because a position of the spacer is not varied, so that adisplay device with little display unevenness can be obtained.

A process to be performed to the first substrate 2001 is described.

First, a first insulating film 2002 is formed over the first substrate2001 by a sputtering method, a printing method, a coating method, or thelike. The first insulating film 2002 has a function of preventing changein characteristics of the transistor due to an impurity from the firstsubstrate 2001 which affects a semiconductor layer. Note that the firstinsulating film 2002 is not necessarily formed when a quartz substrateis used as the first substrate 2001.

Next, a first conductive layer 2003 is formed over the first insulatingfilm 2002 by photolithography, a laser direct writing method, an inkjetmethod, or the like.

Next, a second insulating film 2004 is formed over the entire surface bya sputtering method, a printing method, a coating method, or the like.The second insulating film 2004 has a function of preventing change incharacteristics of the transistor due to an impurity from the firstsubstrate 2001 which affects the semiconductor layer.

Next, a first semiconductor layer 2005 and a second semiconductor layer2006 are formed. Note that the first semiconductor layer 2005 and thesecond semiconductor layer 2006 are formed sequentially and shapesthereof are processed at the same time.

Next, a second conductive layer 2007 is formed by photolithography, alaser direct writing method, an inkjet method, or the like. Note that asa method for etching which is performed at the time of processing ashape of the second conductive layer 2007, dry etching is preferable.Note that either a light-transmitting material or a reflective materialmay be used for the second conductive layer 2007.

Next, a channel region of the transistor is formed. Here, an example ofa step thereof is described. The second semiconductor layer 2006 isetched by using the second conductive layer 2007 as a mask.Alternatively, the second semiconductor layer 2006 may be etched byusing a mask for processing the shape of the second conductive layer2007. Then, the first semiconductor layer 2005 at a position where thesecond semiconductor layer 2006 is removed serves as the channelformation region of the transistor. When the channel region is formed inthis manner, the number of masks can be reduced, so that manufacturingcost can be reduced.

Next, a third insulating film 2008 is formed and a contact hole isformed as selected in the third insulating film 2008. Note that acontact hole may be formed also in the second insulating film 2004 atthe same time as forming the contact hole in the third insulating film2008. Note that a surface of the third insulating film 2008 ispreferably as even as possible. This is because orientation of theliquid crystal molecules is affected by unevenness of a surface withwhich the liquid crystal is in contact, that is, the surface of thethird insulating film 2008.

Next, a third conductive layer 2009 is formed by photolithography, alaser direct writing method, an inkjet method, or the like.

Next, a first alignment film 2010 is formed. Note that after the firstalignment film 2010 is formed, rubbing treatment may be performed so asto control the orientation of the liquid crystal molecules.

The first substrate 2001 which is manufactured as described above andthe second substrate 2016 which is provided with the light-shieldingfilm 2014, the color filter 2015, the fourth conductive layer 2013, thespacer 2017, and the second alignment film 2012 are attached to eachother with a sealant with a gap of several μm therebetween. Then, aliquid crystal material is injected into a space between the twosubstrates. Note that in the TN mode, the fourth conductive layer 2013is provided for the entire surface of the second substrate 2016.

FIG. 29A is an example of a cross-sectional view of a pixel in the casewhere an MVA (multi-domain vertical alignment) mode and a transistor arecombined. By applying the pixel structure shown in FIG. 29A to a liquidcrystal display device of the present invention, a liquid crystaldisplay device having a wider viewing angle characteristics can beobtained.

Features of the pixel structure of the MVA-mode liquid crystal panelshown in FIG. 29A are described. Liquid crystal molecules 2118 shown inFIG. 29A are long and narrow molecules each having a major axis and aminor axis similarly to the liquid crystal molecules 2018. In FIG. 29A,a direction of each of the liquid crystal molecules 2118 is expressed bythe length thereof. That is, each of the liquid crystal molecules 2118shown in FIG. 29A is aligned such that the direction of the major axisis normal to the alignment film. Thus, the liquid crystal molecules 2118at a position where an alignment control protrusion 2119 is formed arealigned radially with the alignment control protrusion 2119 as a center.With this state, a liquid crystal display device having wide viewingangle characteristics can be obtained.

Note that the case is described in which a bottom-gate transistor usingan amorphous semiconductor is used as the transistor. In the case wherea transistor using an amorphous semiconductor is used, a liquid crystaldisplay device can be formed at low cost by using a large substrate.

In FIG. 29A, the two substrates between which the liquid crystal 2111 isinterposed correspond to the first substrate 2101 and the secondsubstrate 2116. A transistor and a pixel electrode are formed over thefirst substrate 2101. A light-shielding film 2114, a color filter 2115,a fourth conductive layer 2113, a spacer 2117, a second alignment film2112, and an alignment control protrusion 2119 are provided for thesecond substrate 2116.

When the light-shielding film 2114 is formed, a display device withlittle light leakage at the time of black display can be obtained. Thelight-shielding film 2114 is not necessarily provided on the secondsubstrate 2116. When the light-shielding film 2114 is not formed, thenumber of steps can be reduced, so that manufacturing cost can bereduced and yield can be improved.

The color filter 2115 is not necessarily provided on the secondsubstrate 2116. When the color filter 2115 is not formed, similarly tothe light-shielding film, the number of steps can be reduced, so thatmanufacturing cost can be reduced and yield can be improved. Note thateven when the color filter 2115 is not formed, a display device whichcan perform color display can be obtained by field sequential driving.

Spherical spacers may be dispersed instead of forming the spacer 2117.When the spherical spacers are dispersed, the number of steps isreduced, so that manufacturing cost can be reduced. In addition, yieldcan be improved. Alternatively, when the spacer 2117 is formed, adistance between the two substrates can be uniform easily because aposition of the spacer is not varied, so that a display device withlittle display unevenness can be obtained.

A process to be performed to the first substrate 2101 is described.

First, a first insulating film 2102 is formed over the first substrate2101 by a sputtering method, a printing method, a coating method, or thelike. The first insulating film 2102 has a function of preventing changein characteristics of the transistor due to an impurity from the firstsubstrate 2101 which affects a semiconductor layer. Note that the firstinsulating film 2102 is not necessarily formed when a quartz substrateis used as the first substrate 2101.

Next, a first conductive layer 2103 is formed over the first insulatingfilm 2102 by photolithography, a laser direct writing method, an inkjetmethod, or the like.

Next, a second insulating film 2104 is formed over the entire surface bya sputtering method, a printing method, a coating method, or the like.The second insulating film 2104 has a function of preventing change incharacteristics of the transistor due to an impurity from the firstsubstrate 2101 which affects the semiconductor layer.

Next, a first semiconductor layer 2105 and a second semiconductor layer2106 are formed. Note that the first semiconductor layer 2105 and thesecond semiconductor layer 2106 are formed sequentially and shapesthereof are processed at the same time.

Next, a second conductive layer 2107 is formed by photolithography, alaser direct writing method, an inkjet method, or the like. Note that asa method for etching which is performed at the time of processing ashape of the second conductive layer 2107, dry etching is preferable.Note that as the second conductive layer 2107, either alight-transmitting material or a reflective material may be used.

Next, a channel region of the transistor is formed. Here, an example ofa step thereof is described. The second semiconductor layer 2106 isetched by using the second conductive layer 2107 as a mask.Alternatively, the second semiconductor layer 2106 may be etched byusing a mask for processing the shape of the second conductive layer2107. Then, the first semiconductor layer 2105 at a position where thesecond semiconductor layer 2106 is removed serves as the channelformation region of the transistor. When the channel region is formed inthis manner, the number of masks can be reduced, so that manufacturingcost can be reduced.

Next, a third insulating film 2108 is formed and a contact hole isformed as selected in the third insulating film 2108. Note that acontact hole may be formed also in the second insulating film 2104 atthe same time as forming the contact hole in the third insulating film2108.

Next, a third conductive layer 2109 is formed by photolithography, alaser direct writing method, an inkjet method, or the like.

Next, a first alignment film 2110 is formed. Note that after the firstalignment film 2110 is formed, rubbing treatment may be performed so asto control the orientation of the liquid crystal molecules.

The first substrate 2101 which is manufactured as described above andthe second substrate 2116 which is provided with the light-shieldingfilm 2114, the color filter 2115, the fourth conductive layer 2113, thespacer 2117, and the second alignment film 2112 are attached to eachother with a sealant with a gap of several μm therebetween. Then, aliquid crystal material is injected into a space between the twosubstrates.

Note that in the MVA mode, the fourth conductive layer 2113 is providedfor the entire surface of the second substrate 2116. Note that thealignment control protrusion 2119 is formed so as to be in contact withthe fourth conductive layer 2113. The alignment control protrusion 2119preferably has a shape with a smooth curved surface. Thus, orientationdefect of the liquid crystal molecules 2118 caused by the alignmentcontrol protrusion 2119 is reduced. Further, since breaking of thealignment film provided for the alignment control protrusion 2119 can beprevented, a defect of the alignment film caused by breaking of thealignment film can be reduced.

FIG. 29B is an example of a cross-sectional view of a pixel in the casewhere a PVA (patterned vertical alignment) mode and a transistor arecombined. By applying the pixel structure shown in FIG. 29B to a liquidcrystal display device of the present invention, a liquid crystaldisplay device having a wider viewing angle characteristics, highresponse speed, and high contrast can be obtained.

Features of the pixel structure shown in FIG. 29B are described. In FIG.29B, direction of each of the liquid crystal molecules 2148 is expressedby the length thereof, similarly to the liquid crystal molecules 2018shown in FIG. 28. Thus, each of the liquid crystal molecules 2148 shownin FIG. 29B is aligned such that the direction of the major axis isnormal to the alignment film. Thus, the liquid crystal molecules 2148which exist in the periphery of the electrode notch portion 2149 whichis not provided with the fourth conductive layer 2143 are alignedradially with a boundary of the electrode notch portion 2149 and thefourth conductive layer 2143 as a center. With this state, a liquidcrystal display device having a wide viewing angle can be obtained.

Note that the case is described in which a bottom-gate transistor usingan amorphous semiconductor is used as the transistor. In the case wherea transistor using an amorphous semiconductor is used, a liquid crystaldisplay device can be formed at low cost by using a large substrate.

In FIG. 29B, the two substrates between which liquid crystal 2141 isinterposed correspond to the first substrate 2131 and the secondsubstrate 2146. A transistor and a pixel electrode are formed over thefirst substrate 2131. A light-shielding film 2144, a color filter 2145,a fourth conductive layer 2143, a spacer 2147, and a second alignmentfilm 2142 are provided for the second substrate 2146.

When the light-shielding film 2144 is formed, a display device withlittle light leakage at the time of black display can be obtained. Thelight-shielding film 2144 is not necessarily provided on the secondsubstrate 2146. When the light-shielding film 2144 is not formed, thenumber of steps can be reduced, so that manufacturing cost can bereduced and yield can be improved.

The color filter 2145 is not necessarily provided on the secondsubstrate 2146. When the color filter 2145 is not formed, similarly tothe light-shielding film, the number of steps can be reduced, so thatmanufacturing cost can be reduced and yield can be improved. Note thateven when the color filter 2145 is not formed, a display device whichcan perform color display can be obtained by field sequential driving.

Spherical spacers may be dispersed instead of forming the spacer 2147.When the spherical spacers are dispersed, the number of steps isreduced, so that manufacturing cost can be reduced. In addition, yieldcan be improved. Alternatively, when the spacer 2147 is formed, adistance between the two substrates can be uniform easily because aposition of the spacer is not varied, so that a display device withlittle display unevenness can be obtained.

A process to be pertained to the first substrate 2131 is described.

First, a first insulating film 2132 is formed over the first substrate2131 by a sputtering method, a printing method, a coating method, or thelike. The first insulating film 2132 has a function of preventing changein characteristics of the transistor due to an impurity from the firstsubstrate 2131 which affects a semiconductor layer. Note that the firstinsulating film 2132 is not necessarily formed when a quartz substrateis used as the first substrate 2131.

Next, a first conductive layer 2133 is formed over the first insulatingfilm 2132 by photolithography, a laser direct writing method, an inkjetmethod, or the like.

Next, a second insulating film 2134 is formed over the entire surface bya sputtering method, a printing method, a coating method, or the like.The second insulating film 2134 has a function of preventing change incharacteristics of the transistor due to an impurity from the firstsubstrate 2131 which affects the semiconductor layer.

Next, a first semiconductor layer 2135 and a second semiconductor Layer2136 are formed. Note that the first semiconductor layer 2135 and thesecond semiconductor layer 2136 are formed sequentially and shapesthereof are processed at the same time.

Next, a second conductive layer 2137 is formed by photolithography, alaser direct writing method, an inkjet method, or the like. Note that asa method for etching which is performed at the time of processing ashape of the second conductive layer 2137, dry etching is preferable.Note that as the second conductive layer 2137, either alight-transmitting material or a reflective material may be used.

Next, a channel region of the transistor is formed. Here, an example ofa step thereof is described. The second semiconductor layer 2136 isetched by using the second conductive layer 2137 as a mask.Alternatively, the second semiconductor layer 2136 may be etched byusing a mask for processing the shape of the second conductive layer2137. Then, the first semiconductor layer 2135 at a position where thesecond semiconductor layer 2136 is removed serves as the channel regionof the transistor. When the channel region is formed in this manner, thenumber of masks can be reduced, so that manufacturing cost can bereduced.

Next, a third insulating film 2138 is formed and a contact hole isformed as selected in the third insulating film 2138. Note that acontact hole may be formed also in the second insulating film 2134 atthe same time as forming the contact hole in the third insulating film2138.

Next, a third conductive layer 2139 is formed by photolithography, alaser direct writing method, an inkjet method, or the like.

Next, a first alignment film 2140 is formed. Note that after the firstalignment film 2140 is formed, rubbing treatment may be performed so asto control the orientation of the liquid crystal molecules.

The first substrate 2131 which is manufactured as described above andthe second substrate 2146 which is provided with the light-shieldingfilm 2144, the color filter 2145, the fourth conductive layer 2143, thespacer 2147, and the second alignment film 2142 are attached to eachother with a sealant with a gap of several μm therebetween. Then, aliquid crystal material is injected into a space between the twosubstrates.

Note that in the PVA mode, the fourth conductive layer 2143 is patternedand is provided with the electrode notch portion 2149. Although a shapeof the electrode notch portion 2149 is not particularly limited, theelectrode notch portion 2149 preferably has a shape in which a pluralityof rectangles having different directions are combined. Thus, aplurality of regions having different alignment can be formed, so that aliquid crystal display device having a wide viewing angle can beobtained. Note that the fourth conductive layer 2143 at the boundarybetween the electrode notch portion 2149 and the fourth conductive layer2143 preferably has a shape with a smooth slope with respect to thebase. Thus, an orientation defect of the liquid crystal molecules 2148which are adjacent to the slope is reduced. Further, since breaking ofthe alignment film provided over the fourth conductive layer 2143 can beprevented, a defect of the alignment film caused by breaking of thealignment film can be reduced.

FIG. 30A is an example of a cross-sectional view of a pixel in the casewhere an IPS (in-plane-switching) mode and a transistor are combined. Byapplying the pixel structure shown in FIG. 30A to a liquid crystaldisplay device, a liquid crystal display device theoretically having awider viewing angle can be obtained.

Features of the pixel structure shown in FIG. 30A are described. Liquidcrystal molecules 2218 shown in FIG. 30A are long and narrow moleculeseach having a major axis and a minor axis, similarly to the liquidcrystal molecules 2018 shown in FIG. 28. In FIG. 30A, a direction ofeach of the liquid crystal molecules 10318 is expressed by the lengththereof. That is, each of the liquid crystal molecules 2218 shown inFIG. 30A is aligned so that the direction of the major axis thereof isalways horizontal to the substrate. Although FIG. 30A shows alignment ofthe liquid crystal molecules 2218 with no electric field in a regionwhere liquid crystal 2211 exists, when an electric field is applied toeach of the liquid crystal molecules 2218, each of the liquid crystalmolecules 2218 rotates in a horizontal plane as the direction of themajor axis thereof is always horizontal to the substrate. With thisstate, a liquid crystal display device having a wide viewing angle canbe obtained.

Note that the case is described in which a bottom-gate transistor usingan amorphous semiconductor is used as the transistor. In the case wherea transistor using an amorphous semiconductor is used, a liquid crystaldisplay device can be formed at low cost by using a large substrate.

In FIG. 30A, the two substrates between which the liquid crystal 2211 isinterposed correspond to the first substrate 2201 and the secondsubstrate 2216. A transistor and a pixel electrode are formed over thefirst substrate 2201. A light-shielding film 2214, a color filter 2215,a spacer 2217, and a second alignment film 2212 are provided for thesecond substrate.

When the light-shielding film 2214 is formed, a display device withlittle light leakage at the time of black display can be obtained. Thelight-shielding film 2214 is not necessarily provided on the secondsubstrate 2216. When the light-shielding film 2214 is not formed, thenumber of steps is reduced, so that manufacturing cost can be reducedand yield can be improved.

The color filter 2215 is not necessarily provided on the secondsubstrate 2216. When the color filter 2215 is not provided, similarly tothe light-shielding film, the number of steps can be reduced, so thatmanufacturing cost can be reduced and yield can be improved. Note thateven when the color filter 2215 is not formed, a display device whichcan perform color display can be obtained by field sequential driving.

Spherical spacers may be dispersed instead of forming the spacer 2217.When the spherical spacers are dispersed, the number of steps isreduced, so that manufacturing cost can be reduced. In addition, yieldcan be improved. Alternatively, when the spacer 2217 is formed, adistance between the two substrates can be uniform easily because aposition of the spacer is not varied, so that a display device withlittle display unevenness can be obtained.

A process to be performed to the first substrate 2201 is described.

First, a first insulating film 2202 is formed over the first substrate2201 by a sputtering method, a printing method, a coating method, or thelike. The first insulating film 2202 has a function of preventing changein characteristics of the transistor due to an impurity from the firstsubstrate 2201 which affects a semiconductor layer. Note that the firstinsulating film 2202 is not necessarily formed when a quartz substrateis used as the first substrate 2201.

Next, a first conductive layer 2203 is formed over the first insulatingfilm 2202 by photolithography, a laser direct writing method, an inkjetmethod, or the like.

Next, a second insulating film 2204 is formed over the entire surface bya sputtering method, a printing method, a coating method, or the like.The second insulating film 2204 has a function of preventing change incharacteristics of the transistor due to an impurity from the firstsubstrate 2201 which affects the semiconductor layer.

Next, a first semiconductor layer 2205 and a second semiconductor layer2206 are formed. Note that the first semiconductor layer 2205 and thesecond semiconductor layer 2206 are formed sequentially and shapesthereof are processed at the same time.

Next, a second conductive layer 2207 is formed by photolithography, alaser direct writing method, an inkjet method, or the like. Note that asa method for etching which is performed at the time of processing ashape of the second conductive layer 2207, dry etching is preferable.Note that as the second conductive layer 2207, either alight-transmitting material or a reflective material may be used.

Next, a channel region of the transistor is formed. Here, an example ofa step thereof is described. The second semiconductor layer 2206 isetched by using the second conductive layer 2207 as a mask.Alternatively, the second semiconductor layer 2206 is etched by using amask for processing the shape of the second conductive layer 2207. Then,the first semiconductor layer 2205 at a position where the secondsemiconductor layer 2206 is removed serves as the channel region of thetransistor. When the channel region is formed in this manner, the numberof masks can be reduced, so that manufacturing cost can be reduced.

Next, a third insulating film 2208 is formed and a contact hole isformed as selected in the third insulating film 2208. Note that acontact hole may be formed also in the second insulating film 2204 atthe same time as forming the contact hole in the third insulating film2208.

Next, a third conductive layer 2209 is formed by photolithography, alaser direct writing method, an inkjet method, or the like. Here, thethird conductive layer 2209 has a shape in which two comb-shapedelectrodes engage with each other. One of the comb-shaped electrodes iselectrically connected to one of a source electrode and a drainelectrode of the transistor, and the other of the comb-shaped electrodesis electrically connected to a common electrode. Thus, a horizontalelectric field can be effectively applied to the liquid crystalmolecules 2218.

Next, a first alignment film 2210 is formed. Note that after the firstalignment film 2210 is formed, rubbing treatment may be performed so asto control the orientation of the liquid crystal molecules.

The first substrate 2201 which is manufactured as described above andthe second substrate 2216 which is provided with the light-shieldingfilm 2214, the color filter 2215, the spacer 2217, and the secondalignment film 2212 are attached to each other with a sealant with a gapof several put therebetween. Then, a liquid crystal material is injectedinto a space between the two substrates.

FIG. 30B is an example of a cross-sectional view of a pixel in the casewhere an FFS (fringe field switching) mode and a transistor arecombined. By applying the pixel structure shown in FIG. 30B to a liquidcrystal display device of the present invention, a liquid crystaldisplay device having a wider viewing angle can be obtained.

Features of the pixel structure shown in FIG. 30B are described. In FIG.30B, direction of each of the liquid crystal molecules 2248 is expressedby the length thereof in a similar way to the liquid crystal molecules2018. That is, each of the liquid crystal molecules 2248 shown in FIG.30B is aligned so that the direction of the major axis thereof is alwayshorizontal to the substrate. Although FIG. 30B shows alignment of theliquid crystal molecules 2248 with no electric field in a region whereliquid crystal 2241 exists, when an electric field is applied to each ofthe liquid crystal molecules 2248, each of the liquid crystal molecules2248 rotates in a horizontal plane as the direction of the major axisthereof is always horizontal to the substrate. With this state, a liquidcrystal display device having a wide viewing angle can be obtained.

Note that the case is described in which a bottom-gate transistor usingan amorphous semiconductor is used as the transistor. In the case wherea transistor using an amorphous semiconductor is used, a liquid crystaldisplay device can be formed at low cost by using a large substrate.

In FIG. 30B, the two substrates between which the liquid crystal 2241 isinterposed correspond to the first substrate 2231 and the secondsubstrate 2246. A transistor and a pixel electrode are formed over thefirst substrate 2241. A light-shielding film 2244, a color filter 2245,a spacer 2247, and a second alignment film 2242 are provided for thesecond substrate 2246.

When the light-shielding film 2244 is formed, a display device withlittle light leakage at the time of black display can be obtained. Thelight-shielding film 2244 is not necessarily provided on the secondsubstrate 2246. When the light-shielding film 2244 is not formed, thenumber of steps can be reduced, so that manufacturing cost can bereduced and yield can be improved.

The color filter 2245 is not necessarily provided on the secondsubstrate 2246. When the color filter 2245 is not formed, similarly tothe light-shielding film, the number of steps can be reduced, so thatmanufacturing cost can be reduced and yield can be improved. Note thateven when the color filter 2245 is not formed, a display device whichcan perform color display can be obtained by field sequential driving.

Spherical spacers may be dispersed instead of forming the spacer 2247.When the spherical spacers are dispersed, the number of steps isreduced, so that manufacturing cost can be reduced. In addition, yieldcan be improved. Alternatively, when the spacer 2247 is formed, adistance between the two substrates can be uniform easily because aposition of the spacer is not varied, so that a display device withlittle display unevenness can be obtained.

A process to be performed to the first substrate 2231 is described.

First, a first insulating film 2232 is formed over the first substrate2231 by a sputtering method, a printing method, a coating method, or thelike. The first insulating film 2232 has a function of preventing changein characteristics of the transistor due to an impurity from the firstsubstrate 2231 which affects a semiconductor layer. Note that the firstinsulating film 2232 is not necessarily formed when a quartz substrateis used as the first substrate 2231.

Next, a first conductive layer 2233 is formed over the first insulatingfilm 2232 by photolithography, a laser direct writing method, an inkjetmethod, or the like.

Next, a second insulating film 2234 is formed over the entire surface bya sputtering method, a printing method, a coating method, or the like.The second insulating film 2234 has a function of preventing change incharacteristics of the transistor due to an impurity from the firstsubstrate 2231 which affects the semiconductor layer.

Next, a first semiconductor layer 2235 and a second semiconductor layer2236 are formed. Note that the first semiconductor layer 2235 and thesecond semiconductor layer 2236 are formed sequentially and shapesthereof are processed at the same time.

Next, a second conductive layer 2237 is formed by photolithography, alaser direct writing method, an inkjet method, or the like. Note that asa method for etching which is performed at the time of processing ashape of the second conductive layer 2237, dry etching is preferable.Note that as the second conductive layer 2237, either alight-transmitting material or a reflective material may be used.

Next, a channel region of the transistor is formed. Here, an example ofa step thereof is described. The second semiconductor layer 2236 isetched by using the second conductive layer 2237 as a mask.Alternatively, the second semiconductor layer 2236 may be etched byusing a mask for processing the shape of the second conductive layer2237. Then, the first semiconductor layer 2235 at a position where thesecond semiconductor layer 2236 is removed serves as the channel regionof the transistor. When the channel region is formed in this manner, thenumber of masks can be reduced, so that manufacturing cost can bereduced.

Next, a third insulating film 2238 is formed and a contact hole isformed as selected in the third insulating film 2238.

Next, a third conductive layer 2239 is formed by photolithography, alaser direct writing method, an inkjet method, or the like.

Next, a fourth insulating film 2249 is formed and a contact hole isformed as selected in the fourth insulating film 2249.

Next, a fourth conductive layer 2243 is formed by photolithography, alaser direct writing method, an inkjet method, or the like. Here, thefourth conductive layer 2243 is comb-shaped.

Next, a first alignment film 2240 is formed. Note that after the firstalignment film 2240 is formed, rubbing treatment may be performed so asto control the orientation of the liquid crystal molecules.

The first substrate 2231 which is manufactured as described above andthe second substrate 2246 which is provided with the light-shieldingfilm 2244, the color filter 2245, the spacer 2247, and the secondalignment film 2242 are attached to each other with a sealant with a gapof several μm therebetween. Then, a liquid crystal material is injectedinto a space between the two substrates, so that a liquid crystal panelcan be manufactured.

Here, materials which can be used for conductive layers or insulatingfilms are described.

As the first insulating film 2002 in FIG. 28, the first insulating film2102 in FIG. 29A, the first insulating film 2132 in FIG. 29B, the firstinsulating film 2202 in FIG. 30A, or the first insulating film 2232 inFIG. 30B, an insulating film such as a silicon oxide film, a siliconnitride film, or a silicon oxynitride (SiO_(X)N_(y)) film can be used.Alternatively, an insulating film having a stacked-layer structure inwhich two or more of a silicon oxide film, a silicon nitride film, asilicon oxynitride (SiO_(X)N_(y)) film, and the like are combined can beused.

As the first conductive layer 2003 in FIG. 28, the first conductivelayer 2103 in FIG. 29A, the first conductive layer 2133 in FIG. 29B, thefirst conductive layer 2203 in FIG. 30A, or the first conductive layer2233 in FIG. 30B, a conductive material such as Mo, Ti, Al, Nd, Cr, orthe like can be used. Alternatively, a stacked-layer structure in whichtwo or more of conductive materials such as Mo, Ti, Al, Nd, Cr, and thelike are combined can be used.

As the second insulating film 2004 in FIG. 28, the second insulatingfilm 2104 in FIG. 29A, the second insulating film 2134 in FIG. 29B, thesecond insulating film 2204 in FIG. 30A, or the second insulating film2234 in FIG. 30B, a thermal oxide film, a silicon oxide film, a siliconnitride film, a silicon oxynitride film, or the like can be used.Alternatively, a stacked-layer structure in which two or more of athermal oxide film, a silicon oxide film, a silicon nitride film, asilicon oxynitride film, and the like are combined can be used. Notethat a silicon oxide film is preferable in a portion which is in contactwith a semiconductor layer. This is because a trap level at an interfacewith the semiconductor layer decreases when a silicon oxide film isused. Note that a silicon nitride film is preferable in a portion whichis in contact with Mo. This is because a silicon nitride film does notoxidize Mo.

As the first semiconductor layer 2005 in FIG. 28, the firstsemiconductor layer 2105 in FIG. 29A, the first semiconductor layer 2135in FIG. 29B, the first semiconductor layer 2205 in FIG. 30A, or thefirst semiconductor layer 2235 in FIG. 30B, silicon, silicon germanium(SiGe), or the like can be used.

As the second semiconductor layer 2006 in FIG. 28, the secondsemiconductor layer 2106 in FIG. 29A, the second semiconductor layer2136 in FIG. 29B, the second semiconductor layer 2206 in FIG. 30A, orthe second semiconductor layer 2236 in FIG. 30B, silicon or the likeincluding phosphorus can be used.

As a light-transmitting material of the second conductive layer 2007 andthe third conductive layer 2009 in FIG. 28; the second conductive layer2107 and the third conductive layer 2109 in FIG. 29A; the secondconductive layer 2137 and the third conductive layer 2139 in FIG. 29B;the second conductive layer 2207 and the third conductive layer 2209 inFIG. 30A; or the second conductive layer 2237, the third conductivelayer 2239, and a fourth conductive layer 2243 in FIG. 30B, an indiumtin oxide (ITO) film formed by mixing tin oxide into indium oxide, anindium tin silicon oxide (ITSO) film formed by mixing silicon oxide intoindium tin oxide (ITO), an indium zinc oxide (IZO) film formed by mixingzinc oxide into indium oxide, a zinc oxide film, a tin oxide film, orthe like can be used. Note that IZO can be formed by sputtering using atarget in which zinc oxide (ZnO) is mixed into ITO at 2 to 20 wt %.

As a reflective material of the second conductive layer 2007 and thethird conductive layer 2009 in FIG. 28; the second conductive layer 2107and the third conductive layer 2109 in FIG. 29A; the second conductivelayer 2137 and the third conductive layer 2139 in FIG. 29B; the secondconductive layer 2207 and the third conductive layer 2209 in FIG. 30A;or the second conductive layer 2237, the third conductive layer 2239,and the fourth conductive layer 2243 in FIG. 30B, Ti, Mo, Ta, Cr, W, Al,or the like can be used. Alternatively, a two-layer structure in whichAl and Ti, Mo, Ta, Cr, or W are stacked, or a three-layer structure inwhich Al is interposed between metals such as Ti, Mo, Ta, Cr, and W maybe used.

As the third insulating film 2008 in FIG. 28, the third insulating film2108 in FIG. 29A, the third insulating film 2138 in FIG. 29B, the thirdconductive layer 2139 in FIG. 29B, the third insulating film 2208 inFIG. 30A, or the third insulating film 2238 and the fourth insulatingfilm 2249 in FIG. 30B, an inorganic material (e.g., silicon oxide,silicon nitride, or silicon oxynitride), an organic compound materialhaving a low dielectric constant (e.g., a photosensitive ornonphotosensitive organic resin material), or the like can be used.Alternatively, a material including siloxane can be used. Note thatsiloxane is a material in which a skeleton structure is formed by a bondof silicon (Si) and oxygen (O). As a substituent, an organic groupincluding at least hydrogen (e.g., an alkyl group or an aryl group) isused. Alternatively, a fluoro group may be used as the substituent.Further alternatively, the organic group including at least hydrogen andthe fluoro group may be used as the substituent.

As the first alignment film 2010 in FIG. 28, the first alignment film2110 in FIG. 29A, the first alignment film 2140 in FIG. 29B, the firstalignment film 2210 in FIG. 30A, or the first alignment film 2240 inFIG. 30B, a film of a high molecular compound such as polyimide can beused.

Next, the pixel structure in the case where each liquid crystal mode andthe transistor are combined is described with reference to a top view (alayout diagram) of the pixel.

Note that as a liquid crystal mode, a TN (twisted nematic) mode, an IPS(in-plane-switching) mode, an FFS (fringe field switching) mode, an MVA(multi-domain vertical alignment) mode, a PVA (patterned verticalalignment) mode, an ASM (axially symmetric aligned micro-cell) mode, anOCB (optical compensated birefringence) mode, an FLC (ferroelectricliquid crystal) mode, an AFLC (antiferroelectric liquid crystal) mode,or the like can be used.

As the transistor, a thin film transistor (a TFT) including a non-singlecrystalline semiconductor layer typified by amorphous silicon,polycrystalline silicon, microcrystalline (also referred to assemi-amorphous) silicon, or the like can be used.

Note that as a structure of the transistor, a top-gate structure, abottom-gate structure, or the like can be used. A channel-etchedtransistor, a channel-protective transistor, or the like can be used asa bottom-gate transistor.

FIG. 31 is an example of a top view of a pixel in the case where a TNmode and a transistor are combined.

The pixel shown in FIG. 31 includes a first transistor 2304, a secondtransistor 2305, a third transistor including a transistor 2320 and atransistor 2321, a first liquid crystal capacitor, a second liquidcrystal capacity, a first storage capacitor and a second storagecapacitor and is connected to a first scanning line 2300, a secondscanning line 2301, a signal line 2302 and a Cs line 2311. Note thatsince an equivalent circuit diagram of a pixel structure shown in FIG.31 is the same as that shown in FIG. 83, the description of details isomitted.

In FIG. 31, a pixel electrode included in the first liquid crystalcapacitor corresponds to a pixel electrode 2307 and a pixel electrodeincluded in the second liquid crystal capacitor corresponds to a pixelelectrode 2308. The first storage capacitor includes a capacitor line2312 which is connected to a Cs line 2311 outside the pixel portion, asemiconductor layer 2309 which is connected to the pixel electrode 2307and an insulating film which is interposed therebetween. In a similarmanner to the first storage capacitor, the second storage capacitorincludes the capacitor line 2312, a semiconductor layer 2310 which isconnected to the pixel electrode 2308 and an insulating film which isinterposed therebetween. Note that the capacitor line 2312 included inthe first storage capacitor and the second storage capacitor ismanufactured in the same process as the first scanning line 2300 whichincludes the gate electrodes included in the transistors 2304, 2305 and2320; the second scanning line 2301 which includes the gate electrodeincluded in the transistor 2321; the Cs line 2311; and the semiconductorlayers 2309 and 2310 which includes source regions, drain regions orchannel formation regions included in the transistors 2304, 2305, 2320and 2321. As the insulating film included in the first storage capacitorand the second storage capacitor, a film which is manufactured in thesame process as the gate insulating films included in the transistors2304, 2305, 2320 and 2321 can be used.

As shown in FIG. 31, a liquid crystal display device which is superiorin viewing angle characteristic can be obtained by using the two liquidcrystal capacitors in which alignment states of liquid crystal moleculesare different from each other. Note that a top view shown in FIG. 31 isan example, the present invention is not limited thereto.

Note that channel width can be widened by employing a structure in whichone of the source electrode and the drain electrode surrounds the otherof the source electrode and the drain electrode in each transistor. Sucha structure is particularly effective when an amorphous semiconductorlayer with lower mobility than that of a crystalline semiconductor layeris used for a semiconductor layer of a transistor included in the pixel.

Note that although this embodiment mode is described with reference tovarious drawings, the contents (or part of the contents) described ineach drawing can be freely applied to, combined with, or replaced withthe contents (or part of the contents) described in another drawing.Further, much more drawings can be formed by combining each part withanother part in the above-described drawings.

Similarly, the contents (or part of the contents) described in eachdrawing in this embodiment mode can be freely applied to, combined with,or replaced with the contents (or part of the contents) described in adrawing in another embodiment mode or embodiment. Further, much moredrawings can be formed by combining each part in each drawing in thisembodiment mode with part of another embodiment mode or embodiment.

Note that this embodiment mode shows examples of embodying, slightlytransforming, partially modifying, improving, describing in detail, orapplying the contents (or part of the contents) described in otherembodiment modes and embodiments, an example of related part thereof, orthe like. Therefore, the contents described in other embodiment modesand embodiments can be freely applied to, combined with, or replacedwith this embodiment mode.

[Embodiment Mode 7]

In this embodiment mode, various liquid crystal modes are described withreference to cross-sectional views.

FIGS. 32A and 32B are schematic views of cross sections of a TN mode.

A liquid crystal layer 3300 is held between a first substrate 3301 and asecond substrate 3302 which are provided so as to be opposite to eachother. A first electrode 3305 is formed on a top surface of the firstsubstrate 3301. A second electrode 3306 is formed on a top surface ofthe second substrate 3302. A first polarizing plate 3303 is provided ona surface of the first substrate 3301, which does not face the liquidcrystal layer. A second polarizing plate 3304 is provided on a surfaceof the second substrate 3302, which does not face the liquid crystallayer. Note that the first polarizing plate 3303 and the secondpolarizing plate 3304 are provided so as to be in a cross nicol state.

The first polarizing plate 3303 may be provided on the top surface ofthe first substrate 3301. The second polarizing plate 3304 may beprovided on the top surface of the second substrate 3302.

It is acceptable as long as at least one of (or both) the firstelectrode 3305 and the second electrode 3306 has a light-transmittingproperty (a transmissive or reflective liquid crystal display device).Alternatively, both the first electrode 3305 and the second electrode3306 may have light-transmitting properties, and part of one of theelectrodes may have reflectivity (a semi-transmissive liquid crystaldisplay device).

FIG. 32A is a schematic view of a cross section in the case wherevoltage is applied to the first electrode 3305 and the second electrode3306 (referred to as a vertical electric field mode). Since liquidcrystal molecules are aligned longitudinally, light emitted from abacklight is not affected by birefringence of the liquid crystalmolecules. In addition, since the first polarizing plate 3303 and thesecond polarizing plate 3304 are provided so as to be in a cross nicolstate, light emitted from the backlight cannot pass through thesubstrate. Therefore, black display is performed.

FIG. 32B is a schematic view of a cross section in the case wherevoltage is not applied to the first electrode 3305 and the secondelectrode 3306. Since the liquid crystal molecules are aligned laterallyand rotated between the first electrode 3305 and the second electrode3306, light emitted from a backlight is affected by birefringence of theliquid crystal molecules. In addition, since the first polarizing plate3303 and the second polarizing plate 3304 are provided so as to be in across nicol state, light emitted from the backlight passes through thesubstrate. Therefore, white display is performed. This is a so-callednormally white mode.

Note that by controlling voltage applied to the first electrode 3305 andthe second electrode 3306, conditions of the liquid crystal molecules,that is, alignment of the liquid crystal molecules can be controlled.Therefore, since the amount of light emitted from the backlight can becontrolled by the liquid crystal molecules, predetermined image displaycan be performed.

A liquid crystal display device having a structure shown in FIG. 32A orFIG. 32B can perform full-color display by being provided with a colorfilter. The color filter can be provided on a first substrate 3301 sideor a second substrate 3302 side.

It is acceptable as long as a known material is used as a liquid crystalmaterial used for a TN mode.

FIGS. 33A and 33B are schematic views of cross sections of a VA mode. Inthe VA mode, liquid crystal molecules are aligned such that they arevertical to a substrate when there is no electric field.

A liquid crystal layer 3400 is held between a first substrate 3401 and asecond substrate 3402 which are provided so as to be opposite to eachother. A first electrode 3405 is formed on a top surface of the firstsubstrate 3401. A second electrode 3406 is formed on a top surface ofthe second substrate 3402. A first polarizing plate 3403 is provided ona surface of the first substrate 3401, which does not face the liquidcrystal layer. A second polarizing plate 3404 is provided on a surfaceof the second substrate 3402, which does not face the liquid crystallayer. Note that the first polarizing plate 3403 and the secondpolarizing plate 3404 are provided so as to be in a cross nicol state.

The first polarizing plate 3403 may be provided on the top surface ofthe first substrate 3401. The second polarizing plate 3404 may beprovided on the top surface of the second substrate 3402.

It is acceptable as long as at least one of (or both) the firstelectrode 3405 and the second electrode 3406 has a light-transmittingproperty (a transmissive or reflective liquid crystal display device).Alternatively, both the first electrode 3405 and the second electrode3406 may have light-transmitting properties, and part of one of theelectrodes may have reflectivity (a semi-transmissive liquid crystaldisplay device).

FIG. 33A is a schematic view of a cross section in the case wherevoltage is applied to the first electrode 3405 and the second electrode3406 (referred to as a vertical electric field mode). Since liquidcrystal molecules are aligned laterally, light emitted from a backlightis affected by birefringence of the liquid crystal molecules. Inaddition, since the first polarizing plate 3403 and the secondpolarizing plate 3404 are provided so as to be in a cross nicol state,light emitted from the backlight passes through the substrate.Therefore, white display is performed.

FIG. 33B is a schematic view of a cross section in the case wherevoltage is not applied to the first electrode 3405 and the secondelectrode 3406. Since liquid crystal molecules are alignedlongitudinally, light emitted from a backlight is not affected bybirefringence of the liquid crystal molecules. In addition, since thefirst polarizing plate 3403 and the second polarizing plate 3404 areprovided so as to be in a cross nicol state, light emitted from thebacklight does not pass through the substrate. Therefore, black displayis performed. This is a so-called normally black mode.

Note that by controlling voltage applied to the first electrode 3405 andthe second electrode 3406, conditions of the liquid crystal molecules,that is, alignment of the liquid crystal molecules can be controlled.Therefore, since the amount of light emitted from the backlight can becontrolled by the liquid crystal molecules, predetermined image displaycan be performed.

A liquid crystal display device having a structure shown in FIG. 33A orFIG. 33B can perform full-color display by being provided with a colorfilter. The color filter can be provided on a first substrate 3401 sideor a second substrate 3402 side.

It is acceptable as long as a known material is used as a liquid crystalmaterial used for a VA mode.

FIGS. 33C and 33D are schematic views of cross sections of an MVA mode.In the MVA mode, viewing angle dependency of each portion is compensatedby each other.

A liquid crystal layer 3410 is held between a first substrate 3411 and asecond substrate 3412 which are provided so as to be opposite to eachother. A first electrode 3415 is formed on a top surface of the firstsubstrate 3411. A second electrode 3416 is formed on a top surface ofthe second substrate 3412. A first protrusion 3417 for controllingalignment is formed on the first electrode 3415. A second protrusion3418 for controlling alignment is formed on the second electrode 3416. Afirst polarizing plate 3413 is provided on a surface of the firstsubstrate 3411, which does not face the liquid crystal layer. A secondpolarizing plate 3414 is provided on a surface of the second substrate3412, which does not face the liquid crystal layer. Note that the firstpolarizing plate 3413 and the second polarizing plate 3414 are providedso as to be in a cross nicol state.

The first polarizing plate 3413 may be provided on the top surface ofthe first substrate 3411. The second polarizing plate 3414 may beprovided on the top surface of the second substrate 3412.

It is acceptable as long as at least one of (or both) the firstelectrode 3415 and the second electrode 3416 has a light-transmittingproperty (a transmissive or reflective liquid crystal display device).Alternatively, both the first electrode 3415 and the second electrode3416 may have light-transmitting properties, and part of one of theelectrodes may have reflectivity (a semi-transmissive liquid crystaldisplay device).

FIG. 33C is a schematic view of a cross section in the case wherevoltage is applied to the first electrode 3415 and the second electrode3416 (referred to as a vertical electric field mode). Light emitted froma backlight is affected by birefringence of the liquid crystalmolecules. In addition, since the first polarizing plate 3413 and thesecond polarizing plate 3414 are provided so as to be in a cross nicolstate, light emitted from the backlight passes through the substrate.Therefore, white display is performed. Further, liquid crystal moleculesare affected by the first protrusion 3417 and the second protrusion 3418and thus aligned so as to tilt toward the first protrusion 3417 and thesecond protrusion 3418. Therefore, viewing angle characteristics can befurther enhanced.

FIG. 33D is a schematic view of a cross section in the case wherevoltage is not applied to the first electrode 3415 and the secondelectrode 3416. Since liquid crystal molecules are alignedlongitudinally, light emitted from a backlight is not affected bybirefringence of the liquid crystal molecules. In addition, since thefirst polarizing plate 3413 and the second polarizing plate 3414 areprovided so as to be in a cross nicol state, light emitted from thebacklight does not pass through the substrate. Therefore, black displayis performed. This is a so-called normally black mode.

Note that by controlling voltage applied to the first electrode 3415 andthe second electrode 3416, conditions of the liquid crystal molecules,that is, alignment of the liquid crystal molecules can be controlled.Therefore, since the amount of light emitted from the backlight can becontrolled by the liquid crystal molecules, predetermined image displaycan be performed.

A liquid crystal display device having a structure shown in FIG. 33C orFIG. 33D can perform full-color display by being provided with a colorfilter. The color filter can be provided on a first substrate 3411 sideor a second substrate 3412 side.

It is acceptable as long as a known material is used as a liquid crystalmaterial used for an MVA mode.

FIGS. 34A and 34B are schematic views of cross sections of an OCB mode.In the OCB mode, viewing angle dependency is low because alignment ofliquid crystal molecules in a liquid crystal layer is opticallycompensated. This state of the liquid crystal molecules is referred toas bend alignment.

A liquid crystal layer 3500 is held between a first substrate 3501 and asecond substrate 3502 which are provided so as to be opposite to eachother. A first electrode 3505 is formed on a top surface of the firstsubstrate 3501. A second electrode 3506 is formed on a top surface ofthe second substrate 3502. A first polarizing plate 3503 is provided ona surface of the first substrate 3501, which does not face the liquidcrystal layer. A second polarizing plate 3504 is provided on a surfaceof the second substrate 3502, which does not face the liquid crystallayer. Note that the first polarizing plate 3503 and the secondpolarizing plate 3504 are provided so as to be in a cross nicol state.

The first polarizing plate 3503 may be provided on the top surface ofthe first substrate 3501. The second polarizing plate 3504 may beprovided on the top surface of the second substrate 3502.

It is acceptable as long as at least one of (or both) the firstelectrode 3505 and the second electrode 3506 has a light-transmittingproperty (a transmissive or reflective liquid crystal display device).Alternatively, both the first electrode 3505 and the second electrode3506 may have light-transmitting properties, and part of one of theelectrodes may have reflectivity (a semi-transmissive liquid crystaldisplay device).

FIG. 34A is a schematic view of a cross section in the case wherevoltage is applied to the first electrode 3505 and the second electrode3506 (referred to as a vertical electric field mode). Since liquidcrystal molecules are aligned longitudinally, light emitted from abacklight is not affected by birefringence of the liquid crystalmolecules. In addition, since the first polarizing plate 3503 and thesecond polarizing plate 3504 are provided so as to be in a cross nicolstate, light emitted from the backlight does not pass through thesubstrate. Therefore, black display is performed.

FIG. 34B is a schematic view of a cross section in the case wherevoltage is not applied to the first electrode 3505 and the secondelectrode 3506. Since liquid crystal molecules are in a bend alignmentstate, light emitted from a backlight is affected by birefringence ofthe liquid crystal molecules. In addition, since the first polarizingplate 3503 and the second polarizing plate 3504 are provided so as to bein a cross nicol state, light emitted from the backlight passes throughthe substrate. Therefore, white display is performed. This is aso-called normally white mode.

Note that by controlling voltage applied to the first electrode 3505 andthe second electrode 3506, conditions of the liquid crystal molecules,that is, alignment of the liquid crystal molecules can be controlled.Therefore, since the amount of light emitted from the backlight can becontrolled by the liquid crystal molecules, predetermined image displaycan be performed.

A liquid crystal display device having a structure shown in FIG. 34A orFIG. 34B can perform full-color display by being provided with a colorfilter. The color filter can be provided on a first substrate 3501 sideor a second substrate 3502 side.

It is acceptable as long as a known material is used as a liquid crystalmaterial used for an OCB mode.

FIGS. 34C and 34D are schematic views of cross sections of an FLC modeor an AFLC mode.

A liquid crystal layer 3510 is held between a first substrate 3511 and asecond substrate 3512 which are provided so as to be opposite to eachother. A first electrode 3515 is formed on a top surface of the firstsubstrate 3511. A second electrode 3516 is formed on a top surface ofthe second substrate 3512. A first polarizing plate 3513 is provided ona surface of the first substrate 3511, which does not face the liquidcrystal layer. A second polarizing plate 3514 is provided on a surfaceof the second substrate 3512, which does not face the liquid crystallayer. Note that the first polarizing plate 3513 and the secondpolarizing plate 3514 are provided so as to be in a cross nicol state.

The first polarizing plate 3513 may be provided on the top surface ofthe first substrate 3511. The second polarizing plate 3514 may beprovided on the top surface of the second substrate 3512.

It is acceptable as long as at least one of or both the first electrode3515 and the second electrode 3516 have light-transmitting properties (atransmissive or reflective liquid crystal display device).Alternatively, both the first electrode 3515 and the second electrode3516 may have light-transmitting properties, and part of one of theelectrodes may have reflectivity (a semi-transmissive liquid crystaldisplay device).

FIG. 34C is a schematic view of a cross section in the case wherevoltage is applied to the first electrode 3515 and the second electrode3516 (referred to as a vertical electric field mode). Since liquidcrystal molecules are aligned laterally in a direction which is deviatedfrom a rubbing direction, light emitted from a backlight is affected bybirefringence of the liquid crystal molecules. In addition, since thefirst polarizing plate 3513 and the second polarizing plate 3514 areprovided so as to be in a cross nicol state, light emitted from thebacklight passes through the substrate. Therefore, white display isperformed.

FIG. 34D is a schematic view of a cross section in the case wherevoltage is not applied to the first electrode 3515 and the secondelectrode 3516. Since liquid crystal molecules are aligned laterally ina rubbing direction, light emitted from a backlight is not affected bybirefringence of the liquid crystal molecules. In addition, since thefirst polarizing plate 3513 and the second polarizing plate 3514 areprovided so as to be in a cross nicol state, light emitted from thebacklight does not pass through the substrate. Therefore, black displayis performed. This is a so-called normally black mode.

Note that by controlling voltage applied to the first electrode 3515 andthe second electrode 3516, conditions of the liquid crystal molecules,that is, alignment of the liquid crystal molecules, can be controlled.Therefore, since the amount of light emitted from the backlight can becontrolled by the liquid crystal molecules, predetermined image displaycan be performed.

A liquid crystal display device having a structure shown in FIG. 34C orFIG. 34D can perform full-color display by being provided with a colorfilter. The color filter can be provided on a first substrate 3511 sideor a second substrate 3512 side.

It is acceptable as long as a known material is used as a liquid crystalmaterial used for an FLC mode or an AFLC mode.

FIGS. 35A and 35B are schematic views of cross sections of an IPS mode.In the IPS mode, the liquid crystal molecules are constantly rotated ina plane parallel to a substrate, and a horizontal electric field mode inwhich electrodes are provided only on one substrate side is used.

A liquid crystal layer 3600 is held between a first substrate 3601 and asecond substrate 3602 which are provided so as to be opposite to eachother. A first electrode 3605 and a second electrode 3606 are formed ona top surface of the second substrate 3602. A first polarizing plate3603 is provided on a surface of the first substrate 3601, which doesnot face the liquid crystal layer. A second polarizing plate 3604 isprovided on a surface of the second substrate 3602, which does not facethe liquid crystal layer. Note that the first polarizing plate 3603 andthe second polarizing plate 3604 are provided so as to be in a crossnicol state.

The first polarizing plate 3603 may be provided on the top surface ofthe first substrate 3601. The second polarizing plate 3604 may beprovided on the top surface of the second substrate 3602.

It is acceptable as long as at least one of (or both) the firstelectrode 3605 and the second electrode 3606 has a light-transmittingproperty (a transmissive or reflective liquid crystal display device).Alternatively, both the first electrode 3605 and the second electrode3606 may have light-transmitting properties, and part of one of theelectrodes may have reflectivity (a semi-transmissive liquid crystaldisplay device).

FIG. 35A is a schematic view of a cross section in the case wherevoltage is applied to the first electrode 3605 and the second electrode3606. Since liquid crystal molecules are aligned along a line ofelectric force which is deviated from a rubbing direction, light emittedfrom a backlight is affected by birefringence of the liquid crystalmolecules. In addition, since the first polarizing plate 3603 and thesecond polarizing plate 3604 are provided so as to be in a cross nicolstate, light emitted from the backlight passes through the substrate.Therefore, white display is performed.

FIG. 35B is a schematic view of a cross section in the case wherevoltage is not applied to the first electrode 3605 and the secondelectrode 3606. Since liquid crystal molecules are aligned laterally ina rubbing direction, light emitted from a backlight is not affected bybirefringence of the liquid crystal molecules. In addition, since thefirst polarizing plate 3603 and the second polarizing plate 3604 areprovided so as to be in a cross nicol state, light emitted from thebacklight does not pass through the substrate. Therefore, black displayis performed. This is a so-called normally black mode.

By controlling voltage applied to the first electrode 3605 and thesecond electrode 3606, conditions of the liquid crystal molecules, thatis, alignment of the liquid crystal molecules can be controlled.Therefore, since the amount of light emitted from the backlight can becontrolled by the liquid crystal molecules, predetermined image displaycan be performed.

A liquid crystal display device having a structure shown in FIG. 35A orFIG. 35B can perform full-color display by being provided with a colorfilter. The color filter can be provided on a first substrate 3601 sideor a second substrate 3602 side.

It is acceptable as long as a known material is used as a liquid crystalmaterial used for an IPS mode.

FIGS. 35C and 35D are schematic views of cross sections of an FFS mode.Also in the FFS mode, the liquid crystal molecules are constantlyrotated in a plane parallel to a substrate, and a horizontal electricfield method in which electrodes are provided only on one substrate sideis used.

A liquid crystal layer 3610 is held between a first substrate 3611 and asecond substrate 3612 which are provided so as to be opposite to eachother. A second electrode 3616 is formed on a top surface of the secondsubstrate 3612. An insulating film 3617 is formed on a top surface ofthe second electrode 3616. A first electrode 3615 is formed over theinsulating film 3617. A first polarizing plate 3613 is provided on asurface of the first substrate 3611, which does not face the liquidcrystal layer. A second polarizing plate 3614 is provided on a surfaceof the second substrate 3612, which does not face the liquid crystallayer. Note that the first polarizing plate 3613 and the secondpolarizing plate 3614 are provided so as to be in a cross nicol state.

The first polarizing plate 3613 may be provided on the top surface ofthe first substrate 3611. The second polarizing plate 3614 may beprovided on the top surface of the second substrate 3612.

It is acceptable as long as at least one of (or both) the firstelectrode 3615 and the second electrode 3616 has a light-transmittingproperty (a transmissive or reflective liquid crystal display device).Alternatively, both the first electrode 3615 and the second electrode3616 may have light-transmitting properties, and part of one of theelectrodes may have reflectivity (a semi-transmissive liquid crystaldisplay device).

FIG. 35C is a schematic view of a cross section in the case wherevoltage is applied to the first electrode 3615 and the second electrode3616. Since liquid crystal molecules are aligned along a line ofelectric force which is deviated from a rubbing direction, light emittedfrom a backlight is affected by birefringence of the liquid crystalmolecules. In addition, since the first polarizing plate 3613 and thesecond polarizing plate 3614 are provided so as to be in a cross nicolstate, light emitted from the backlight passes through the substrate.Therefore, white display is performed.

FIG. 35D is a schematic view of a cross section in the case wherevoltage is not applied to the first electrode 3615 and the secondelectrode 3616. Since liquid crystal molecules are aligned laterally ina rubbing direction, light emitted from the backlight is not affected bybirefringence of the liquid crystal molecules. In addition, since thefirst polarizing plate 3613 and the second polarizing plate 3614 areprovided so as to be in a cross nicol state, light emitted from thebacklight does not pass through the substrate. Therefore, black displayis performed. This is a so-called normally black mode.

Note that by controlling voltage applied to the first electrode 3615 andthe second electrode 3616, conditions of the liquid crystal molecules,that is, alignment of the liquid crystal molecules can be controlled.Therefore, since the amount of light emitted from the backlight can becontrolled by the liquid crystal molecules, predetermined image displaycan be performed.

A liquid crystal display device having a structure shown in FIG. 35C orFIG. 35D can perform full-color display by being provided with a colorfilter. The color filter can be provided on a first substrate 3611 sideor a second substrate 3612 side.

It is acceptable as long as a known material is used as a liquid crystalmaterial used for an FFS mode.

Next, various liquid crystal modes are described with reference to topplan views.

FIG. 36 is a top plan view of one of a plurality of liquid crystalcapacitors included in a pixel to which an MVA mode is applied.

FIG. 36 shows a first electrode 3701, second electrodes (3702 a, 3702 b,and 3702 c), and a protrusion 3703. The first electrode 3701 is formedover the entire surface of a counter substrate. The protrusion 3703 isformed so as to have boomerang shapes. The second electrodes (3702 a,3702 b, and 3702 c) are formed over the first electrode 3701 so as tohave shapes corresponding to the protrusion 3703.

Opening portions of the second electrodes (3702 a, 3702 b, and 3702 c)function like protrusions.

In the case where voltage is applied to the first electrode 3701 and thesecond electrodes (3702 a, 3702 b, and 3702 c) (referred to as avertical electric field mode), liquid crystal molecules are aligned soas to tilt toward the opening portions of the second electrodes (3702 a,3702 b, and 3702 c) and the protrusion 3703. Therefore, viewing anglecharacteristics can be enhanced. Note that since light emitted from abacklight passes through a substrate when a pair of polarizing plates isprovided so as to be in a cross nicol state, white display is performed.

In the case where voltage is not applied to the first electrode 3701 andthe second electrodes (3702 a, 3702 b, and 3702 c), the liquid crystalmolecules are aligned longitudinally. Since light emitted from thebacklight does not pass through a panel when the pair of polarizingplates is provided so as to be in the cross nicol state, black displayis performed. This is a so-called normally black mode.

Note that by controlling voltage applied to the first electrode 3701 andthe second electrodes (3702 a, 3702 b, and 3702 c), conditions of theliquid crystal molecules, that is, alignment of the liquid crystalmolecules can be controlled. Therefore, since the amount of lightemitted from the backlight can be controlled by the liquid crystalmolecules, predetermined image display can be performed.

It is acceptable as long as a known material is used as a liquid crystalmaterial used for an MVA mode.

FIGS. 37A to 37D are top plan views of a liquid crystal capacitor towhich an IPS mode is applied. In the IPS mode, the liquid crystalmolecules are constantly rotated in a plane parallel to a substrate, anda horizontal electric field mode in which electrodes are provided onlyon one substrate side is used.

In the IPS mode, a pair of electrodes is formed so as to have differentshapes.

FIG. 37A shows a first electrode 3801 and a second electrode 3802. Thefirst electrode 3801 and the second electrode 3802 have wavy shapes.

FIG. 37B shows a first electrode 3811 and a second electrode 3812. Thefirst electrode 3811 and the second electrode 3812 have shapes havingconcentric openings.

FIG. 37C shows a first electrode 3821 and a second electrode 3822. Thefirst electrode 3821 and the second electrode 3822 have comb shapes andpartially overlap with each other.

FIG. 37D shows a first electrode 3831 and a second electrode 3832. Thefirst electrode 3831 and the second electrode 3832 have comb shapes inwhich electrodes engage with each other.

In the case where voltage is applied to the first electrodes (3801,3811, 3821, and 3831) and the second electrodes (3802, 3812, 3822, and3832), liquid crystal molecules are aligned along a line of electricforce which is deviated from a rubbing direction. Since light emittedfrom a backlight passes through a substrate when a pair of polarizingplates is provided so as to be in a cross nicol state, white display isperformed.

In the case where voltage is not applied to the first electrodes (3801,3811, 3821, and 3831) and the second electrodes (3802, 3812, 3822, and3832), the liquid crystal molecules are aligned laterally in the rubbingdirection. Since light emitted from the backlight does not pass throughthe substrate when the pair of polarizing plates is provided so as to bein the cross nicol state, black display is performed. This is aso-called normally black mode.

Note that by controlling voltage applied to the first electrodes and thesecond electrodes, conditions of the liquid crystal molecules, that is,alignment of the liquid crystal molecules can be controlled. Therefore,since the amount of light emitted from the backlight can be controlledby the liquid crystal molecules, predetermined image display can beperformed.

It is acceptable as long as a known material be used as a liquid crystalmaterial used for an IPS mode.

FIGS. 38A to 38D are top plan views of a liquid crystal capacitor towhich an FFS mode is applied. In the FFS mode, the liquid crystalmolecules are constantly rotated in a plane parallel to a substrate, anda horizontal electric field method in which electrodes are provided onlyon one substrate side is used.

In the FFS mode, a first electrode is formed over a top surface of asecond electrode so as to have various shapes.

FIG. 38A shows a first electrode 3901 and a second electrode 3902. Thefirst electrode 3901 has a bent boomerang shape. The second electrode3902 is not necessarily patterned.

FIG. 38B shows a first electrode 3911 and a second electrode 3912. Thefirst electrode 3911 has a concentric shape. The second electrode 3912is not necessarily patterned.

FIG. 38C shows a first electrode 3921 and a second electrode 3922. Thefirst electrode 3921 has a comb shape in which electrodes engage witheach other. The second electrode 3922 is not necessarily patterned.

FIG. 38D shows a first electrode 3931 and a second electrode 3932. Thefirst electrode 3931 has a comb shape. The second electrode 3932 is notnecessarily patterned.

In the case where voltage is applied to the first electrodes (3901,3911, 3921, and 3931) and the second electrodes (3902, 3912, 3922, and3932), liquid crystal molecules are aligned along a line of electricforce which is deviated from a rubbing direction. Since light emittedfrom a backlight passes through a substrate when a pair of polarizingplates is provided so as to be in a cross nicol state, white display isperformed.

In the case where voltage is not applied to the first electrodes (3901,3911, 3921, and 3931) and the second electrodes (3902, 3912, 3922, and3932), the liquid crystal molecules are aligned laterally in the rubbingdirection. Since light emitted from the backlight does not pass throughthe substrate when the pair of polarizing plates is provided so as to bein the cross nicol state, black display is performed. This is aso-called normally black mode.

Note that by controlling voltage applied to the first electrodes and thesecond electrodes, conditions of the liquid crystal molecules, that is,alignment of the liquid crystal molecules can be controlled. Therefore,since the amount of light emitted from the backlight can be controlledby the liquid crystal molecules, predetermined image display can beperformed.

It is acceptable as long as a known material is used as a liquid crystalmaterial used for an FFS mode.

Although this embodiment mode is described with reference to variousdrawings, the contents (or may be part of the contents) described ineach drawing can be freely applied to, combined with, or replaced withthe contents (or may be part of the contents) described in anotherdrawing. Further, even more drawings can be formed by combining eachpart with another part in the above-described drawings.

Similarly, the contents (or may be part of the contents) described ineach drawing of this embodiment mode can be freely applied to, combinedwith, or replaced with the contents (or may be part of the contents)described in a drawing in another embodiment mode. Further, even moredrawings can be formed by combining each part with part of anotherembodiment mode in the drawings of this embodiment mode.

Note that this embodiment mode shows an example of an embodied case ofthe contents (or may be part of the contents) described in otherembodiment modes, an example of slight transformation thereof, anexample of partial modification thereof, an example of improvementthereof, an example of detailed description thereof, an applicationexample thereof, an example of related part thereof, or the like.Therefore, the contents described in other embodiment modes can befreely applied to, combined with, or replaced with this embodiment mode.

[Embodiment Mode 8]

In this embodiment mode, a peripheral portion of a liquid crystal panelis described.

FIG. 39 shows an example of a liquid crystal display device including aso-called edge-light type backlight unit 2601 and a liquid crystal panel2607. An edge-light type corresponds to a type in which a light sourceis provided at an end of a backlight unit and fluorescence of the lightsource is emitted from the entire light-emitting surface. The edge-lighttype backlight unit is thin and can save power.

The backlight unit 2601 includes a diffusion plate 2602, a light guideplate 2603, a reflection plate 2604, a lamp reflector 2605, and a lightsource 2606.

The light source 2606 has a function of emitting light as necessary. Forexample, as the light source 2606, a cold cathode fluorescent lamp, ahot cathode fluorescent lamp, a light-emitting diode, an inorganic EL,an organic EL, or the like is used. The lamp reflector 2605 has afunction of efficiently guiding fluorescence from the light source 2606to the light guide plate 2603. The light guide plate 2603 has a functionof guiding light to the entire surface by total reflection offluorescence from the light source 2606. The diffusion plate 2602 has afunction of reducing variations in brightness. The reflection plate 2604has a function of reflecting light which is leaked from the light guideplate 2603 to a direction which is opposite to the liquid crystal panel2607 to be reused.

Note that a control circuit for controlling luminance of the lightsource 2606 is connected to the backlight unit 2601. By using thiscontrol circuit, luminance of the light source 2606 can be controlled.

FIGS. 40A to 40D are views each showing a detailed structure of theedge-light type backlight unit. Note that description of a diffusionplate, a light guide plate, a reflection plate, and the like is omitted.

A backlight unit 2701 shown in FIG. 40A has a structure in which a coldcathode fluorescent lamp 2703 is used as a light source. In addition, alamp reflector 2702 is provided to efficiently reflect light from thecold cathode fluorescent lamp 2703. Such a structure is often used for alarge display device.

A backlight unit 2711 shown in FIG. 40B has a structure in whichlight-emitting diodes (LEDs) 2713 are used as light sources. Forexample, the light-emitting diodes (W) 2713 which emit white light areprovided at a predetermined interval. In addition, a lamp reflector 2712is provided to efficiently reflect light from the light-emitting diodes2713.

Since emission intensity of light-emitting diodes is high, a structureusing light-emitting diodes is suitable for a large display device.Since light-emitting diodes are superior in color reproducibility, anaccurate image can be displayed with respect to inputted imageinformation. Since light-emitting diodes are small, an arrangement areacan be reduced. Therefore, a frame of a display device can be narrowed.

Note that in the case where light-emitting diodes are mounted on a largedisplay device, the light-emitting diodes can be provided on a back sideof the substrate. The light-emitting diodes of R, G, and B aresequentially provided at a predetermined interval.

A backlight unit 2721 shown in FIG. 40C has a structure in whichlight-emitting diodes (LEDs) 2723, light-emitting diodes (LEDs) 2724,and light-emitting diodes (LEDs) 2725 of R, G, and B are used as lightsources. The light-emitting diodes (LEDs) 2723, the light-emittingdiodes (LEDs) 2724, and the light-emitting diodes (LEDs) 2725 of R, G,and B are each provided at a predetermined interval. By using thelight-emitting diodes (LEDs) 2723, the light-emitting diodes (LEDs)2724, and the light-emitting diodes (LEDs) 2725 of R, G, and B, colorreproducibility can be improved. In addition, a lamp reflector 2722 isprovided to efficiently reflect light from the light-emitting diodes.

Since luminance of light-emitting diodes is high, a structure usinglight-emitting diodes of R, G, and B as light sources is suitable for alarge display device. Since light-emitting diodes are superior in colorreproducibility, an accurate image can be displayed with respect toinputted image information. Since light-emitting diodes are small, anarrangement area can be reduced. Therefore, a frame of a display devicecan be narrowed.

By sequentially making the light-emitting diodes of R, G, and B emitlight in accordance with time, color display can be performed. This is aso-called field sequential mode.

Note that a light-emitting diode which emits white light can be combinedwith the light-emitting diodes (LEDs) 2723, the light-emitting diodes(LEDs) 2724, and the light-emitting diodes (LEDs) 2725 of R, G, and B.

Note that in the case where light-emitting diodes are mounted on a largedisplay device, the light-emitting diodes can be provided on a back sideof the substrate. The light-emitting diodes of R, G, and B aresequentially provided at a predetermined interval. By providing thelight-emitting diodes in this manner, color reproducibility can beimproved.

A backlight unit 2731 shown in FIG. 40D has a structure in whichlight-emitting diodes (LEDs) 2733, light-emitting diodes (LEDs) 2734,and light-emitting diodes (LEDs) 2735 of R, G and B are used as lightsources. For example, among the light-emitting diodes (LEDs) 2733, thelight-emitting diodes (LEDs) 2734, and the light-emitting diodes (LEDs)2735 of R, G, and B, a plurality of the light-emitting diodes (LEDs)2734 of a color with low emission intensity (e.g., green) are providedin FIG. 40D. With such a structure, color reproducibility can beimproved. In addition, a lamp reflector 2732 is provided to efficientlyreflect light from the light-emitting diodes.

Since emission intensity of light-emitting diodes is high, a structureusing light-emitting diodes of R, G, and B as light sources is suitablefor a large display device. Since light-emitting diodes are superior incolor reproducibility, an accurate image can be displayed with respectto inputted image information. Since light-emitting diodes are small, anarrangement area can be reduced. Therefore, a frame of a display devicecan be narrowed.

By sequentially making the light-emitting diodes of R, G, and B emitlight in accordance with time, color display can be performed.

Note that a light-emitting diode which emits white light can be combinedwith the light-emitting diodes (LEDs) 2733, the light-emitting diodes(LEDs) 2734, and the light-emitting diodes (LEDs) 2735 of R, G, and B.

Note that in the case where light-emitting diodes are mounted on a largedisplay device, the light-emitting diodes can be provided on a back sideof the substrate. The light-emitting diodes of R, G, and B aresequentially provided at a predetermined interval. By providing thelight-emitting diodes in this manner, color reproducibility can beimproved.

FIG. 41A shows an example of a liquid crystal display device including aso-called direct-type backlight unit 2800 and a liquid crystal panel2805. A direct type corresponds to a type in which a light source isprovided directly under a light-emitting surface and fluorescence of thelight source is emitted from the entire light-emitting surface. Thedirect-type backlight unit can efficiently utilize the amount of emittedlight.

A backlight unit 2800 includes a diffusion plate 2801, a light-shieldingplate 2802, a lamp reflector 2803, and a light source 2804.

The light source 2804 has a function of emitting light as necessary. Forexample, as the light source 2804, a cold cathode fluorescent lamp, ahot cathode fluorescent lamp, a light-emitting diode, an inorganic ELelement, an organic EL element, or the like is used. The lamp reflector2803 has a function of efficiently guiding fluorescence from the lightsource 2804 to the diffusion plate 2801 and the light-shielding plate2802. The light-shielding plate 2802 has a function of reducingvariations in luminance by shielding much light as light becomes moreintense in accordance with provision of the light source 2804. Thediffusion plate 2801 also has a function of reducing variations inluminance.

A control circuit for controlling luminance of the light source 2804 isconnected to the backlight unit 2800. By using this control circuit,luminance of the light source 2804 can be controlled.

FIG. 41B shows an example of a liquid crystal display device including aso-called direct-type backlight unit 2810 and a liquid crystal panel2815.

A backlight unit 2810 includes a diffusion plate 2811; a light-shieldingplate 2812; a lamp reflector 2813; and a light source (R) 2814 a, alight source (G) 2814 b, and a light source (B) 2814 c of R, G, and B.

Each of the light source (R) 2814 a, the light source (G) 2814 b, andthe light source (B) 2814 c of R, G, and B has a function of emittinglight as necessary. For example, as each of the light source (R) 2814 a,the light source (G) 2814 b, and the light source (B) 2814 c, a coldcathode fluorescent lamp, a hot cathode fluorescent lamp, alight-emitting diode, an inorganic EL element, an organic EL element, orthe like is used. The lamp reflector 2813 has a function of efficientlyguiding fluorescence from the light sources 2814 a to 2814 c to thediffusion plate 2811 and the light-shielding plate 2812. Thelight-shielding plate 2812 has a function of reducing variations inbrightness or luminance by shielding much light as light becomes moreintense in accordance with provision of the light sources 2814 a to 2814c. The diffusion plate 2811 also has a function of reducing variationsin brightness or luminance.

A control circuit for controlling luminance of the light source (R) 2814a, the light source (G) 2814 b, and the light source (B) 2814 c of R, G,and B is connected to the backlight unit 2810. By using this controlcircuit, luminance of the light source (R) 2814 a, the light source (G)2814 b, and the light source (B) 2814 c of R, G, and B can becontrolled.

FIG. 42 shows an example of a structure of a polarizing plate (alsoreferred to as a polarizing film).

A polarizing film 2900 includes a protective film 2901, a substrate film2902, a PVA polarizing film 2903, a substrate film 2904, an adhesivelayer 2905, and a mold release film 2906.

The PVA polarizing film 2903 has a function of generating light in onlya certain vibration direction (linear polarized light). Specifically,the PVA polarizing film 2903 includes molecules, which function as apolarizer in which lengthwise electron density and widthwise electrondensity are greatly different from each other. The PVA polarizing film2903 can generate linear polarized light by uniforming directions of themolecules in which lengthwise electron density and widthwise electrondensity are greatly different from each other.

For example, a high molecular film of poly vinyl alcohol is doped withan iodine compound and a PVA film is pulled in a certain direction, sothat a film in which iodine molecules are aligned in a certain directioncan be obtained as the PVA polarizing film 2903. Then, light which isparallel to a major axis of the iodine molecule is absorbed by theiodine molecule. Note that a dichroic dye may be used instead of iodinefor high durability use and high heat resistance use. Note that it ispreferable that the dye be used for a liquid crystal display devicewhich needs to have durability and heat resistance, such as an in-carLCD or an LCD for a projector.

When the PVA polarizing film 2903 is sandwiched by films to be basematerials (the substrate film 2902 and the substrate film 2904) fromboth sides, reliability can be improved. Note that the PVA polarizingfilm 2903 may be sandwiched by triacetylcellulose (TAC) films with highlight-transmitting properties and high durability. Note that each of thesubstrate films and the TAC films function as protective layers ofpolarizer included in the PVA polarizing film 2903.

The adhesive layer 2905 which is to be attached to a glass substrate ofthe liquid crystal panel is attached to one of the substrate films (thesubstrate film 2904). Note that the adhesive layer 2905 is formed byapplying an adhesive to one of the substrate films (the substrate film2904). The mold release film 2906 (a separate film) is provided to theadhesive layer 2905.

The protective film 2901 is provided to the other of the substratesfilms (the substrate film 2902).

A hard coating scattering layer (an anti-glare layer) may be provided ona surface of the polarizing film 2900. Since the surface of the hardcoating scattering layer has minute unevenness formed by AG treatmentand has an anti-glare function which scatters external light, reflectionof external light in the liquid crystal panel can be prevented. Surfacereflection can also be prevented.

Note that a treatment in which plurality of optical thin film layershaving different refractive indexes are layered (also referred to asanti-reflection treatment or AR treatment) may be performed on thesurface of the polarizing film 2900. The plurality of layered opticalthin film layers having different refractive indexes can reducereflectivity on the surface by an interference effect of light.

FIGS. 43A to 43C each show an example of a system block of the liquidcrystal display device.

As shown in FIG. 43A, in a pixel portion 3005, signal lines 3012 whichare extended from a signal line driver circuit 3003 are provided. Inaddition, in the pixel portion 3005, scan lines 3010 which are extendedfrom a scan line driver circuit 3004 are also provided. In addition, aplurality of pixels are arranged in matrix in cross regions of thesignal lines 3012 and the scan lines 3010. Note that each of theplurality of pixels includes a switching element. Here, detaileddescription of the pixel portion 3005 is omitted since it is describedin the above-mentioned embodiment mode.

In FIG. 43A, a driver circuit portion 3008 includes a control circuit3002, the signal line driver circuit 3003, and the scan line drivercircuit 3004. An image signal is input to the control circuit 3002. Thesignal line driver circuit 3003 and the scan Line driver circuit 3004are controlled by the control circuit 3002 in accordance with thiscontrol signal 3001. That is, the control circuit 3002 inputs a controlsignal to each of the signal line driver circuit 3003 and the scan linedriver circuit 3004. Then, in accordance with this control signal, thesignal line driver circuit 20403 inputs a video signal to each of thesignal lines 3012 and the scan line driver circuit 3004 inputs a scansignal to each of the scan lines 3010. Then, the switching elementincluded in the pixel is selected in accordance with the scan signal andthe video signal is input to a pixel electrode of the pixel.

Note that the control circuit 3002 also controls a power source 3007 inaccordance with the control signal 3001. The power source 3007 includesa unit for supplying power to a lighting unit 3006. As the lighting unit3006, an edge-light type backlight unit or a direct-type backlight unitcan be used. Note that a front light may be used as the lighting unit3006. A front light corresponds to a plate-like lighting unit includinga luminous body and a light conducting body, which is attached to thefront surface side of a pixel portion and illuminates the whole area. Byusing such a lighting unit, the pixel portion can be uniformlyilluminated at low power consumption.

As shown in FIG. 43B, the scan line driver circuit 3004 includes a shiftregister 3041, a level shifter 3042, and a circuit functioning as abuffer 3043. A signal such as a gate start pulse (GSP) or a gate clocksignal (GCK) is input from the control circuit 3002 to the shiftregister 3041.

As shown in FIG. 43C, the signal line driver circuit 3003 includes ashift register 3031, a first latch 3032, a second latch 3033, a levelshifter 3034, and a circuit functioning as a buffer 3035. The circuitfunctioning as the buffer 3035 corresponds to a circuit which has afunction of amplifying a weak signal and includes an operationalamplifier or the like. A signal such as a source start pulse (SSP) or asource clock signal (SCK) is input to the level shifter 3034 and data(DATA) such as a video signal is input to the first latch 3032. A latch(LAT) signal can be temporally held in the second latch 3033 and aresimultaneously input to the pixel portion 3005 by a latch (LAT) signal.This is referred to as line sequential driving. Therefore, when a pixelis used in which not line sequential driving but dot sequential drivingis performed, the second latch can be omitted.

Note that in this embodiment mode, a known liquid crystal panel can beused for the liquid crystal panel. For example, a structure in which aliquid crystal layer is sealed between two substrates can be used as theliquid crystal panel. A transistor, a capacitor, a pixel electrode, analignment film, or the like is formed over one of the substrates. Apolarizing plate, a retardation plate, or a prism sheet may be providedover the surface opposite to a top surface of the one of the substrates.A color filter, a black matrix, a counter electrode, an alignment film,or the like is provided over the other of the substrates. A polarizingplate or a retardation plate may be provided over the surface oppositeto a top surface of the other of the substrates. The color filter andthe black matrix may be formed over the top surface of the one of thesubstrates. Note that three-dimensional display can be performed byproviding a slit (a grid) over the top surface side of the one of thesubstrates or the surface opposite to the top surface side of the one ofthe substrates.

Each of the polarizing plate, the retardation plate, and the prism sheetcan be provided between the two substrates. Alternatively, each of thepolarizing plate, the retardation plate, and the prism sheet can beintegrated with one of the two substrates.

Note that although this embodiment mode is described with reference tovarious drawings, the contents (or may be part of the contents)described in each drawing can be freely applied to, combined with, orreplaced with the contents (or may be part of the contents) described inanother drawing. Further, even more drawings can be formed by combiningeach part with another part in the above-described drawings.

Similarly, the contents (or may be part of the contents) described ineach drawing of this embodiment mode can be freely applied to, combinedwith, or replaced with the contents (or may be part of the contents)described in a drawing in another embodiment mode or embodiment.Further, even more drawings can be formed by combining each part withpart of another embodiment mode or embodiment in the drawings of thisembodiment mode.

This embodiment mode shows an example of an embodied case of thecontents (or may be part of the contents) described in other embodimentmodes and embodiments, an example of slight transformation thereof, anexample of partial modification thereof, an example of improvementthereof, an example of detailed description thereof, an applicationexample thereof, an example of related part thereof, or the like.Therefore, the contents described in other embodiment modes andembodiments can be freely applied to, combined with, or replaced withthis embodiment mode.

[Embodiment Mode 9]

In this embodiment mode, a method for driving a display device isdescribed. In particular, a method for driving a liquid crystal displaydevice is described.

A liquid crystal display panel which can be used for a liquid crystaldisplay device described in this embodiment mode has a structure inwhich a liquid crystal material is interposed between two substrates.Each of the two substrates is provided with an electrode for controllingan electric field applied to the liquid crystal material. A liquidcrystal material corresponds to a material optical and electricalproperties of which are changed by an electric field externally applied.Accordingly, a liquid crystal panel corresponds to a device in whichdesired optical and electrical properties can be obtained by controllingvoltage applied to the liquid crystal material with use of the electrodeincluded in each of the two substrates. In addition, many electrodes arearranged in a planar manner so that each of the electrodes correspondsto a pixel, and voltages applied to the pixels are individuallycontrolled; therefore, a liquid crystal display panel which can displaya high-definition image can be obtained.

Here, response time of the liquid crystal material due to change in anelectric field depends on a space (a cell gap) between the twosubstrates and a type or the like of the liquid crystal material, and isgenerally several milliseconds to several ten milliseconds. When theamount of change in the electric field is small, the response time ofthe liquid crystal material is further lengthened. This characteristiccauses defects in image display, such as an after image, a phenomenon inwhich traces can be seen, and decrease in contrast when the liquidcrystal panel displays a moving image. In particular, when a half toneis changed into another half tone (when change in the electric field issmall), a degree of the above-described defects become noticeable.

On the other hand, as a particular problem of a liquid crystal panelusing an active matrix method, fluctuation in writing voltage due toconstant charge driving is given. Constant charge driving in thisembodiment mode is described below.

A pixel circuit using an active matrix method includes a switch whichcontrols writing and a capacitor which holds a charge. A method fordriving the pixel circuit using the active matrix method corresponds toa method in which predetermined voltage is written in a pixel circuitwith a switch in an on state, and immediately after that, the switch isturned off and a charge in the pixel circuit is held (a hold state). Atthe time of the hold state, exchange of the charge between inside andoutside of the pixel circuit is not performed (a constant charge). Ingeneral, a period when the switch is in an off state is approximatelyseveral hundreds (the number of scan lines) of times longer than aperiod when the switch is in an on state. Accordingly, it is likely thatthe switch of the pixel circuit is almost always in an off state. Asdescribed above, constant charge driving in this embodiment modecorresponds to a driving method in which a pixel circuit is in a holdstate in almost all periods when a liquid crystal panel is driven.

Next, electrical properties of the liquid crystal material aredescribed. A dielectric constant as well as optical properties of theliquid crystal material are changed when an electric field externallyapplied is changed. That is, when it is considered that each pixel ofthe liquid crystal panel is a capacitor (a liquid crystal element)interposed between two electrodes, the capacitor corresponds to acapacitor, capacitance of which is changed in accordance with appliedvoltage. This phenomenon is called dynamic capacitance.

When a capacitor, the capacitance of which is changed in accordance withapplied voltage in this manner, is driven by the constant chargedriving, the following problem occurs. When capacitance of a liquidcrystal element is changed in a hold state in which a charge is notmoved, applied voltage is also changed. This can be understood from thefact that the amount of charges is constant in a relational expressionof (the amount of charges)=(capacitance)×(applied voltage).

For the above-described reasons, voltage at the time of a hold state ischanged from voltage at the time of writing because constant chargedriving is performed in a liquid crystal panel using an active matrixmethod. Accordingly, change in transmittance of the liquid crystalelement is different from change in transmittance of a liquid crystalelement in a driving method which does not take a hold state. FIGS. 44Ato 44C show this state. FIG. 44A shows an example of controlling voltagewritten in a pixel circuit when time is represented by a horizontal axisand an absolute value of the voltage is represented by a vertical axis.FIG. 44B shows an example of controlling voltage written in the pixelcircuit when time is represented by a horizontal axis and the voltage isrepresented by a vertical axis. FIG. 44C shows change in transmittanceof the liquid crystal element over time in the case where the voltageshown in FIG. 44A or 44B is written in the pixel circuit when time isrepresented by a horizontal axis and transmittance of the liquid crystalelement is represented by a vertical axis. In each of FIGS. 44A to 44C,a period F indicates a period for rewriting the voltage, and time forrewriting the voltage is denoted by t1, t2, t3, t4, and the like.

Here, writing voltage corresponding to image data input to the liquidcrystal display device corresponds to |V1| in rewriting at the time of 0and corresponds to |V2| in rewriting at the time of t1, t2, t3, t4, andthe like (see FIG. 44A).

Note that polarity of the writing voltage corresponding to image datainput to the liquid crystal display device may be switched periodically(inversion driving: see FIG. 44B). Since direct voltage can be preventedfrom being applied to a liquid crystal as much as possible by using thismethod, burn-in or the like caused by deterioration of the liquidcrystal element can be prevented. Note that a period of switching thepolarity (an inversion period) may be the same as a period of rewritingvoltage. In this case, generation of a flicker caused by inversiondriving can be reduced because the inversion period is short. Further,the inversion period may be a period which is integral times the periodof rewriting voltage. In this case, power consumption can be reducedbecause the inversion period is long and frequency of writing voltagecan be decreased by changing the polarity.

FIG. 44C shows change in transmittance of the liquid crystal elementover time when voltage as shown in FIG. 44A or 44B is applied to theliquid crystal element. Here, the voltage |V1| is applied to the liquidcrystal element, and transmittance of the liquid crystal element afterenough time passes corresponds to TR1. Similarly, the voltage |V2| isapplied to the liquid crystal element, and transmittance of the liquidcrystal element after enough time passes corresponds to TR2. When thevoltage applied to the liquid crystal element is changed from |V1| to|V2| at the time of t1, transmittance of the liquid crystal element doesnot immediately become TR2 but slowly changes as shown by a dashed line30401. For example, when the period of rewriting voltage is the same asa frame period (16.7 milliseconds) of an image signal of 60 Hz, time forseveral frames is necessary until transmittance is changed to TR2.

Note that smooth change in transmittance over time as shown in thedashed line 30401 corresponds to change in transmittance over time whenthe voltage |V2| is accurately applied to the liquid crystal element. Inan actual liquid crystal panel, for example, in a liquid crystal panelusing an active matrix method, transmittance of the liquid crystalelement does not changed over time as shown by the dashed line 30401 butgradually changes over time as shown by a solid line 30402. This isbecause voltage at the time of a hold state is changed from voltage atthe time of writing due to constant charge driving, and it is impossibleto reach intended voltage only by one writing. Accordingly, the responsetime of transmittance of the liquid crystal element becomes furtherlonger than original response time (the clashed line 30401) inappearance, so that defects when an image is displayed, such as an afterimage, a phenomenon in which traces can be seen, or decrease in contrastnoticeably occur.

By using overdriving, it is possible to solve a phenomenon in which theresponse time in appearance becomes further longer because of shortageof writing by dynamic capacitance and constant charge driving as well aslength of the original response time of the liquid crystal element.FIGS. 45A to 45C show this state. FIG. 45A shows an example ofcontrolling voltage written in a pixel circuit when time is representedby a horizontal axis and an absolute value of the voltage is representedby a vertical axis. FIG. 45B shows an example of controlling voltagewritten in the pixel circuit when time is represented by a horizontalaxis and the voltage is represented by a vertical axis. FIG. 45C showschange in transmittance of the liquid crystal element over time in thecase where the voltage shown in FIG. 45A or 45B is written in the pixelcircuit when time is represented by a horizontal axis and transmittanceof the liquid crystal element is represented by a vertical axis. In eachof FIGS. 45A to 45C, a period F indicates a period for rewriting thevoltage, and time for rewriting the voltage is denoted by t1, t2, t3,t4, and the like.

Here, writing voltage corresponding to image data input to the liquidcrystal display device corresponds to |V1| in rewriting at the time of0, corresponds to |V3| in rewriting at the time of t1, and correspondsto |V2| in rewriting at the time of t2, t3, t4, and the like (see FIG.45A).

Note that polarity of the writing voltage corresponding to image datainput to the liquid crystal display device may be switched periodically(inversion driving: see FIG. 45B). Since direct voltage can be preventedfrom being applied to a liquid crystal as much as possible by using thismethod, burn-in or the like caused by deterioration of the liquidcrystal element can be prevented. Note that a period of switching thepolarity (an inversion period) may be the same as a period of rewritingvoltage. In this case, generation of a flicker caused by inversiondriving can be reduced because the inversion period is short. Further,the inversion period may be a period which is integral times the periodof rewriting voltage. In this case, power consumption can be reducedbecause the inversion period is long and frequency of writing voltagecan be decreased by changing the polarity.

FIG. 45C shows change in transmittance of the liquid crystal elementover time when voltage as shown in FIG. 45A or 45B is applied to theliquid crystal element. Here, the voltage |V1| is applied to the liquidcrystal element and transmittance of the liquid crystal element afterenough time passes corresponds to TR1. Similarly, the voltage |V2| isapplied to the liquid crystal element and transmittance of the liquidcrystal element after enough time passes corresponds to TR2. Similarly,the voltage |V3| is applied to the liquid crystal element andtransmittance of the liquid crystal element after enough time passescorresponds to TR3. When the voltage applied to the liquid crystalelement is changed from |V1| to |V3| at the time of t1, transmittance ofthe liquid crystal element is tried to be changed to TR3 for severalframes as shown by a dashed line 30501. However, application of thevoltage |V3| is terminated at the time of t2, and the voltage |V2| isapplied after the time of t2. Therefore, transmittance of the liquidcrystal element does not become as shown by the dashed line 30501 butbecomes as shown by a solid line 30502. It is preferable that a value ofthe voltage |V3| be set so that transmittance is approximately TR2 atthe time of t2. Here, the voltage |V3| is also referred to asoverdriving voltage.

That is, the response time of the liquid crystal element can becontrolled to some extent by changing |V3|, which is the overdrivingvoltage. This is because the response time of the liquid crystal elementis changed by the strength of an electric field. Specifically, theresponse time of the liquid crystal element becomes shorter as theelectric field is stronger, and the response time of the liquid crystalelement becomes longer as the electric field is weaker.

It is preferable that |V3|, which is the overdriving voltage, be changedin accordance with the amount of change in the voltage, that is, thevoltage |V1| and the voltage |V2| which provide intended transmittanceTR1 and TR2. This is because appropriate response time can be alwaysobtained by changing |V3|, which is the overdriving voltage, inaccordance with change in the response time of the liquid crystalelement even when the response time of the liquid crystal element ischanged by the amount of change in the voltage.

It is preferable that |V3|, which is the overdriving voltage, be changeddepending on a mode of the liquid crystal element, such as a TN mode, aVA mode, an IPS mode, or an OCB mode. This is because appropriateresponse time can be always obtained by changing |V3|, which is theoverdriving voltage, in accordance with change in the response time ofthe liquid crystal element even when the response time of the liquidcrystal element is changed depending on the mode of the liquid crystalelement.

Note that the voltage rewriting period F may be the same as a frameperiod of an input signal. In this case, a liquid crystal display devicewith low manufacturing cost can be obtained since a peripheral drivercircuit of the liquid crystal display device can be simplified.

Note that the voltage rewriting period F may be shorter than the frameperiod of the input signal. For example, the voltage rewriting period Fmay be one half the frame period of the input signal, or one third orless the frame period of the input signal. It is effective to combinethis method with a measure against deterioration in quality of a movingimage caused by hold driving of the liquid crystal display device, suchas black data insertion driving, backlight blinking, backlight scanning,or intermediate image insertion driving by motion compensation. That is,since required response time of the liquid crystal element is short inthe measure against deterioration in quality of a moving image caused byhold driving of the liquid crystal display device, the response time ofthe liquid crystal element can be relatively shortened easily by usingthe overdriving method described in this embodiment mode. Although theresponse time of the liquid crystal element can be shortened by a cellgap, a liquid crystal material, a mode of the liquid crystal element, orthe like, it is technically difficult to shorten the response time ofthe liquid crystal element. Therefore, it is very important to use amethod for shortening the response time of the liquid crystal element bya driving method, such as overdriving.

Note also that the voltage rewriting period F may be longer than theframe period of the input signal. For example, the voltage rewritingperiod F may be twice the frame period of the input signal, or threetimes or more the frame period of the input signal. It is effective tocombine this method with a means (a circuit) which determines whethervoltage is not rewritten for a long period or not. That is, when thevoltage is not rewritten for a long period, an operation of the circuitcan be stopped during a period where no voltage is rewritten withoutperforming a rewriting operation of the voltage. Thus, a liquid crystaldisplay device with low power consumption can be obtained.

Next, a specific method for changing the overdriving voltage |V3| inaccordance with the voltage |V1| and the voltage |V2|, which provideintended transmittance TR1 and TR2, is described.

Since an overdriving circuit corresponds to a circuit for appropriatelycontrolling the overdriving voltage |V3| in accordance with the voltage|V1| and the voltage |V2|, which provide intended transmittance TR1 andTR2, signals input to the overdriving circuit are a signal related tothe voltage |V1|, which provides intended transmittance TR1, and asignal related to the voltage |V2|, which provides intendedtransmittance TR2; and a signal output from the overdriving circuit is asignal related to the overdriving voltage |V3|. Here, each of thesesignals may have an analog voltage value such as the voltage (|V1|,|V2|, or |V3|) applied to the liquid crystal element or may be a digitalsignal for supplying the voltage applied to the liquid crystal element.Here, the signal related to the overdriving circuit is described as adigital signal.

First, a general structure of the overdriving circuit is described withreference to FIG. 46A. Here, input image signals 3101 a and 3101 b areused as signals for controlling the overdriving voltage. As a result ofprocessing these signals, an output image signal 3104 is to be output asa signal which provides the overdriving voltage.

Since the voltage |V1| and the voltage |V2|, which provide intendedtransmittance TR1 and TR2, are image signals in adjacent frames, it ispreferable that the input image signals 3101 a and 3101 b be also imagesignals in adjacent frames. In order to obtain such signals, the inputimage signal 3101 a is input to a delay circuit 3102 in FIG. 46A, and asignal which is consequently output can be used as the input imagesignal 310 lb. An example of the delay circuit 3102 includes a memory.That is, the input image signal 3101 a is stored in the memory in orderto delay the input image signal 3101 a for one frame, and at the sametime, a signal stored in the previous frame is extracted from the memoryas the input image signal 3101 b, and the input image signals 3101 a and3101 b are simultaneously input to a correction circuit 3103. Thus, theimage signals in adjacent frames can be handled. By inputting the imagesignals in adjacent frames to the correction circuit 3103, the outputimage signal 3104 can be obtained. Note that when a memory is used asthe delay circuit 3102, a memory having capacity for storing an imagesignal for one frame in order to delay the input image signal 3101 a forone frame (i.e., a frame memory) can be obtained. Thus, the memory canhave a function as a delay circuit without causing excess and deficiencyof memory capacity.

Next, the delay circuit 3102 formed mainly for reducing memory capacityis described. Since memory capacity can be reduced by using such acircuit as the delay circuit 3102, manufacturing cost can be reduced.

Specifically, a delay circuit as shown in FIG. 46B can be used as thedelay circuit 3102 having such characteristics. The delay circuit shownin FIG. 46B includes an encoder 3105, a memory 3106, and a decoder 3107.

Operations of the delay circuit 3102 shown in FIG. 46B are as follows.First, compression processing is performed by the encoder 3105 beforethe input image signal 3101 a is stored in the memory 3106. Thus, thesize of data to be stored in the memory 3106 can be reduced.Accordingly, memory capacity can be reduced, and manufacturing cost canbe reduced. Then, a compressed image signal is transferred to thedecoder 3107 and extension processing is performed here. Thus, thesignal which has been compressed by the encoder 3105 can be restored.Here, compression and extension processing which is performed by theencoder 3105 and the decoder 3107 may be reversible processing.Accordingly, since the image signal does not deteriorate even aftercompression and extension processing is performed, memory capacity canbe reduced without causing deterioration of quality of an image, whichis finally displayed on a device. Alternatively, compression andextension processing which is performed by the encoder 3105 and thedecoder 3107 may be non-reversible processing. Accordingly, since thesize of data of the compressed image signal can be made extremely small,memory capacity can be significantly reduced.

As a method for reducing memory capacity, various methods can be used aswell as the above-described method. For example, a method in which colorinformation included in an image signal is reduced (e.g., tone reductionfrom 260 thousand colors to 65 thousand colors is performed) or theamount of data is reduced (resolution is reduced) without performingimage compression by an encoder can be used.

Next, specific examples of the correction circuit 3103 are describedwith reference to FIGS. 46C to 46E. The correction circuit 3103corresponds to a circuit for outputting an output image signal of acertain value from two input image signals. Here, when a relationbetween the two input image signals and the output image signal isnon-linear and it is difficult to calculate the relation by simpleoperation, a look up table (LUT) may be used as the correction circuit3103. Since the relation between the two input image signals and theoutput image signal is calculated in advance by measurement in a LUT,the output image signal corresponding to the two input image signals canbe calculated only by seeing the LUT (see FIG. 46C). By using a LUT 3108as the correction circuit 3103, the correction circuit 3103 can berealized without complicated circuit design or the like.

Since the LUT is one of memories, it is preferable to reduce memorycapacity as much as possible in order to reduce manufacturing cost. Asan example of the correction circuit 3103 for realizing reduction inmemory capacity, a circuit shown in FIG. 46D can be considered. Thecorrection circuit 3103 shown in FIG. 46D includes a LUT 3109 and anadder 3110. Difference data between the input image signal 3101 a andthe output image signal 3104 to be output is stored in the LUT 3109.That is, corresponding difference data from the input image signal 3101a and the input image signal 3101 b is extracted from the LUT 3109, andthe extracted difference data and the input image signal 3101 a areadded by the adder 3110, so that the output image signal 3104 can beobtained. Note that when data stored in the LUT 3109 is difference data,memory capacity of the LUT can be reduced. This is because the size ofdifference data is smaller than that of the output image signal 3104 asit is, so that memory capacity necessary for the LUT 3109 can bereduced.

In addition, when the output image signal can be calculated by simpleoperation such as four arithmetic operations of the two input imagesignals, the correction circuit 3103 can be realized by combination ofsimple circuits such as an adder, a subtractor, and a multiplier.Accordingly, it is not necessary to use the LUT, and manufacturing costcan be significantly reduced. As such a circuit, a circuit shown in FIG.46E can be considered. The correction circuit 3103 shown in FIG. 46Eincludes a subtractor 3111, a multiplier 3112, and an adder 3113. First,difference between the input image signal 3101 a and the input imagesignal 3101 b is calculated by the subtractor 3111. After that, adifferential value is multiplied by an appropriate coefficient by usingthe multiplier 3112. Then, the differential value multiplied by theappropriate coefficient is added to the input image signal 3101 a by theadder 3113; thus, the output image signal 3104 can be obtained. By usingsuch a circuit, it is not necessary to use the LUT. Therefore,manufacturing cost can be significantly reduced.

By using the correction circuit 3103 shown in FIG. 46E under a certaincondition, inappropriate output of the output image signal 3104 can beprevented. The condition is that the output image signal 3104 applyingthe overdriving voltage and a differential value between the input imagesignals 3101 a and 3101 b have linearity. The slope of this linearity isa coefficient to be multiplied by the multiplier 3112. That is, it ispreferable that the correction circuit 3103 in FIG. 46E be used for aliquid crystal element having such properties. As a liquid crystalelement having such properties, an IPS mode liquid crystal element inwhich response time has little gray-scale dependency is considered. Forexample, when the correction circuit 3103 shown in FIG. 46E is used foran IPS mode liquid crystal element in this manner, manufacturing costcan be significantly reduced and an overdriving circuit which canprevent output of the inappropriate output image signal 3104 can beobtained.

Note that operations which are similar to those of the circuit shown inFIGS. 46A to 46E may be realized by software processing. As the memoryused for the delay circuit, another memory included in the liquidcrystal display device, a memory included in a device which transfers animage displayed on the liquid crystal display device (e.g., a video cardor the like included in a personal computer or a device similar to thepersonal computer), or the like can be used. Accordingly, not only canmanufacturing cost be reduced, intensity of overdriving, availability,or the like can be selected in accordance with user's preference.

Next, driving which controls a potential of a common line is describedwith reference to FIGS. 47A and 47B. FIG. 47A shows a plurality of pixelcircuits in which one common line is provided with respect to one scanline in a display device using a display element which has capacitiveproperties, such as a liquid crystal element. Each of the pixel circuitsshown in FIG. 47A includes a transistor 3201, an auxiliary capacitor3202, a display element 3203, a video signal line 3204, a scan line3205, and a common line 3206.

A gate electrode of the transistor 3201 is electrically connected to thescan line 3205. One of a source electrode and a drain electrode of thetransistor 3201 is electrically connected to the video signal line 3204.The other of the source electrode and the drain electrode of thetransistor 3201 is electrically connected to one electrode of theauxiliary capacitor 3202 and one electrode of the display element 3203.The other electrode of the auxiliary capacitor 3202 is electricallyconnected to the common line 3206.

First, in each pixel selected by the scan line 3205, voltagecorresponding to a video signal is applied to the display element 3203and the auxiliary capacitor 3202 through the video signal line 3204since the transistor 3201 is turned on. At this time, when the videosignal is a signal which makes all of pixels connected to the commonline 3206 display a minimum gray scale or a maximum gray scale, it isnot necessary that the video signal be written in each of the pixelsthrough the video signal line 3204. Voltage applied to the displayelement 3203 can be changed by changing a potential of the common line3206 instead of writing the video signal through the video signal line3204.

Next, FIG. 47B shows a plurality of pixel circuits in which two commonlines are provided with respect to one scan line in a display deviceusing a display element which has capacitive properties, such as aliquid crystal element. Each of the pixel circuits shown in FIG. 47Bincludes a transistor 3211, an auxiliary capacitor 3212, a displayelement 3213, a video signal line 3214, a scan line 3215, a first commonline 3216, and a second common line 3217.

A gate electrode of the transistor 3211 is electrically connected to thescan line 3215. One of a source electrode and a drain electrode of thetransistor 3211 is electrically connected to the video signal line 3214.The other of the source electrode and the drain electrode of thetransistor 3211 is electrically connected to one electrode of theauxiliary capacitor 3212 and one electrode of the display element 3213.The other electrode of the auxiliary capacitor 3212 is electricallyconnected to the first common line 3216. Further, in a pixel which isadjacent to the pixel, the other electrode of the auxiliary capacitor3212 is electrically connected to the second common line 3217.

In the pixel circuits shown in FIG. 47B, the number of pixels which areelectrically connected to one common line is small. Accordingly, bychanging a potential of the first common line 3216 or the second commonline 3217 instead of writing a video signal through the video signalline 3214, frequency of changing voltage applied to the display element3213 is significantly increased. In addition, source inversion drivingor dot inversion driving can be performed. By performing sourceinversion driving or dot inversion driving, reliability of the elementcan be improved and a flicker can be suppressed.

Next, a scanning backlight is described with reference to FIGS. 66A to66C. FIG. 66A shows a scanning backlight in which cold cathodefluorescent lamps are arranged. The scanning backlight shown in FIG. 66Aincludes a diffusion plate 6601 and N pieces of cold cathode fluorescentlamps 6602-1 to 6602-N. The N pieces of the cold cathode fluorescentlamps 6602-1 to 6602-N are arranged on the back side of the diffusionplate 6601, so that the N pieces of the cold cathode fluorescent lamps6602-1 to 6602-N can be scanned while luminance thereof is changed.

Change in luminance of each cold cathode fluorescent lamp in scanning isdescribed with reference to FIG. 66C. First, luminance of the coldcathode fluorescent lamp 6602-1 is changed for a certain period. Afterthat, luminance of the cold cathode fluorescent lamp 6602-2 which isprovided adjacent to the cold cathode fluorescent lamp 6602-1 is changedfor the same period. In this manner, luminance is changed sequentiallyfrom the cold cathode fluorescent lamps 6602-1 to 6602-N. Note thatalthough luminance which is changed for a certain period is set to belower than original luminance in FIG. 66C, it may be higher thanoriginal luminance. In addition, although scanning is performed from thecold cathode fluorescent lamps 6602-1 to 6602-N, scanning may beperformed from the cold cathode fluorescent lamps 6602-N to 6602-1,which is in a reversed order.

By performing driving as in FIG. 66C, average luminance of the backlightcan be decreased. Therefore, power consumption of the backlight, whichmainly takes up power consumption of the liquid crystal display device,can be reduced.

Note that an LED may be used as a light source of the scanningbacklight. FIG. 66B shows the scanning backlight in that case. Thescanning backlight in FIG. 6613 includes a diffusion plate 6611 andlight sources 6612-1 to 6612-N, in each of which LEDs are arranged. Whenthe LED is used as the light source of the scanning backlight, it isadvantageous in that the backlight can be thin and lightweight and thata color reproduction area can be widened. Further, since the LEDs whichare arranged in each of the light sources 6612-1 to 6612-N can besimilarly scanned, a dot scanning backlight can also be obtained. Byusing the dot scanning backlight, image quality of a moving image can befurther improved.

When the LED is used as the light source of the backlight, driving canbe performed by changing luminance as shown in FIG. 66C as well.

Although this embodiment mode is described with reference to variousdrawings, the contents (or part of the contents) described in eachdrawing can be freely applied to, combined with, or replaced with thecontents (or part of the contents) described in another drawing.Further, much more drawings can be formed by combining each part withanother part in the above-described drawings.

Similarly, the contents (or part of the contents) described in eachdrawing in this embodiment mode can be freely applied to, combined with,or replaced with the contents (or part of the contents) described in adrawing in another embodiment mode. Further, much more drawings can beformed by combining each part in each drawing in this embodiment modewith part of another embodiment mode.

This embodiment mode shows examples of embodying, slightly transforming,partially modifying, improving, describing in detailed, or applying thecontents (or part of the contents) described in other embodiment modes,an example of related part thereof, or the like. Therefore, the contentsdescribed in other embodiment modes can be freely applied to, combinedwith, or replaced with this embodiment mode.

[Embodiment Mode 10]

In this embodiment mode, an operation of a display device is described.

FIG. 67 shows a structure example of a display device.

A display device includes a pixel portion 6701, a signal line drivercircuit 6703, and a scan line driver circuit 6704. In the pixel portion6701, a plurality of signal lines S1 to Sn extend from the signal linedriver circuit 6703 in a column direction. In the pixel portion 6701, aplurality of scan lines G1 to Gm extend from the scan line drivercircuit 6704 in a row direction. Pixels 6702 are arranged in matrix ateach intersection of the plurality of signal lines S1 to Sn and theplurality of scan lines G1 to Gm.

The signal line driver circuit 6703 has a function of outputting asignal to each of the signal lines S1 to Sn. This signal may be referredto as a video signal. The scan line driver circuit 6704 has a functionof outputting a signal to each of the scan lines G1 to Gm. This signalmay be referred to as a scan signal.

The pixel 6702 includes at least a switching element connected to thesignal line. On/off of the switching element is controlled by apotential of the scan line (a scan signal). When the switching elementis turned on, the pixel 6702 is selected. On the other hand, when theswitching element is turned off, the pixel 6702 is not selected.

When the pixel 6702 is selected (a selection state), a video signal isinput to the pixel 6702 from the signal line. A state (e.g., luminance,transmittance, or voltage of a storage capacitor) of the pixel 6702 ischanged in accordance with the video signal input thereto.

When the pixel 6702 is not selected (a non-selection state), the videosignal is not input to the pixel 6702. Note that the pixel 6702 holds apotential corresponding to the video signal which is input whenselected; thus, the pixel 6702 maintains the state (e.g., luminance,transmittance, or voltage of a storage capacitor) in accordance with thevideo signal.

Note that a structure of the display device is not limited to that shownin FIG. 67. For example, an additional wiring (such as a scan line, asignal line, a power supply line, a capacitor line, or a common line)may be added in accordance with the structure of the pixel 6702. Asanother example, a circuit having various functions may be added.

FIG. 68 shows an example of a timing chart for describing an operationof a display device.

The timing chart of FIG. 68 shows one frame period corresponding to aperiod when an image of one screen is displayed. One frame period is notparticularly limited, but is at least at least preferably 1/60 second orless so that a viewer does not perceive a flicker.

The timing chart of FIG. 68 shows timing of selecting the scan line G1in the first row, the scan line Gi (one of the scan lines G1 to Gm) inthe i-th row, the scan line Gi+1 in the (i+1)th row, and the scan lineGm in the m-th row.

At the same time as the scan line is selected, the pixel 6702 connectedto the scan line is also selected. For example, when the scan line Gi inthe i-th row is selected, the pixel 6702 connected to the scan line Giin the i-th row is also selected.

The scan lines G1 to Gm are sequentially selected (hereinafter alsoreferred to as scanned) from the scan line G1 in the first row to thescan line Gm in the m-th row. For example, while the scan line Gi in thei-th row is selected, the scan lines (G1 to Gi−1 and Gi+1 to Gm) otherthan the scan line Gi in the i-th row are not selected. Then, during thenext period, the scan line Gi+1 in the (i+1)th row is selected. Notethat a period during which one scan line is selected is referred to asone gate selection period. In addition, the period is also referred toas a writing period in the row selected by the scan line.

Accordingly, when a scan line in a certain row is selected, videosignals from the signal lines S1 to Sn are input to a plurality ofpixels 6702 connected to the scan line, respectively. For example, whilethe scan line Gi in the i-th row is selected, given video signals areinput from the signal lines S1 to Sn to the plurality of pixels 6702connected to the scan line Gi in the i-th row, respectively. Thus, eachof the plurality of pixels 6702 can be controlled individually by thescan signal and the video signal.

Next, the case where one gate selection period is divided into aplurality of subgate selection periods is described. FIG. 69 is a timingchart in the case where one gate selection period is divided into twosubgate selection periods (a first subgate selection period and a secondsubgate selection period).

Note that one gate selection period may be divided into three or moresubgate selection periods.

The timing chart of FIG. 69 shows one frame period corresponding to aperiod when an image of one screen is displayed. One frame period is notparticularly limited, but is at least at least preferably 1/60 second orless so that a viewer does not perceive a flicker.

Note that one frame is divided into two subframes (a first subframe anda second subframe).

The timing chart of FIG. 69 shows timing of selecting the scan line Giin the i-th row, the scan line Gi+1 in the (i+1)th row, the scan line Gj(one of the scan lines Gi+1 to Gm) in the f-th row, and the scan lineGj+1 (one of the scan lines Gi+1 to Gm) in the (j+1)th row.

At the same time as the scan line is selected, the pixel 6702 connectedto the scan line is also selected. For example, when the scan line Gi inthe i-th row is selected, the pixel 6702 connected to the scan line Giin the i-th row is also selected.

The scan lines G1 to Gm are sequentially scanned in each subgateselection period. For example, in one gate selection period, the scanline Gi in the i-th row is selected in the first subgate selectionperiod, and the scan line Gj in the j-th row is selected in the secondsubgate selection period. Thus, in one gate selection period, anoperation can be performed as if the scan signals of two rows areselected. At this time, different video signals are input to the signallines S1 to Sn in the first subgate selection period and the secondsubgate selection period. Accordingly, different video signals can beinput to a plurality of pixels 6702 connected to the i-th row and aplurality of pixels 6702 connected to the j-th row.

Next, a driving method of converting a frame rate of image data to beinput (also referred to as input frame rate) and a frame rate of display(also referred to as a display frame rate) is described. Note that theframe rate is the number of frames per second, and its unit is Hz.

In this embodiment mode, the input frame rate does not necessarilycorrespond to the display frame rate. When the input frame rate and thedisplay frame rate are different from each other, the frame rate can beconverted by a circuit which converts a frame rate of image data (aframe rate conversion circuit). In such a manner, even when the inputframe rate and the display frame rate are different from each other,display can be performed at a variety of display frame rates.

When the input frame rate is higher than the display frame rate, part ofthe image data to be input is discarded and the input frame rate isconverted so that display is performed at a variety of display framerates. In this case, the display frame rate can be reduced; thus,operating frequency of a driver circuit used for display can be reduced,and power consumption can be reduced. On the other hand, when the inputframe rate is lower than the display frame rate, display can beperformed at a variety of converted display frame rates by a method suchas a method in which all or part of the image data to be input isdisplayed more than once, a method in which another image is generatedfrom the image data to be input, or a method in which an image having norelation to the image data to be input is generated. In this case,quality of moving images can be improved by the display frame rate beingincreased.

In this embodiment mode, a frame rate conversion method in the casewhere the input frame rate is lower than the display frame rate isdescribed in detail. Note that a frame rate conversion method in thecase where the input frame rate is higher than the display frame ratecan be realized by performance of the frame rate conversion method inthe case where the input frame rate is lower than the display frame ratein reverse order.

In this embodiment mode, an image displayed at the same frame rate asthe input frame rate is referred to as a basic image. An image which isdisplayed at a frame rate different from that of the basic image anddisplayed to ensure that the input frame rate and the display frame rateare consistent to each other is referred to as an interpolation image.As the basic image, the same image as that of the image data to be inputcan be used. As the interpolation image, the same image as the basicimage can be used. Further, an image different from the basic image canbe generated, and the generated image can be used as the interpolationimage.

In order to generate the interpolation image, the following methods canbe used, for example: a method in which temporal change (movement ofimages) of the image data to be input is detected and an image in anintermediate state between the images is employed as the interpolationimage, a method in which an image obtained by multiplication ofluminance of the basic image by a coefficient is employed as theinterpolation image, and a method in which a plurality of differentimages are generated from the image data to be input and the pluralityof images are continuously displayed (one of the plurality of images isemployed as the basic image and the other images are employed asinterpolation images) so as to allow a viewer to perceive an imagecorresponding to the image data to be input. Examples of the method inwhich a plurality of different images are generated from the image datato be input include a method in which a gamma value of the image data tobe input is converted and a method in which a gray scale value includedin the image data to be input is divided up.

Note that an image in an intermediate state (an intermediate image)refers to an image obtained by detection of temporal change (movement ofimages) of the image data to be input and interpolation of the detectedmovement. Obtaining an intermediate image by such a method is referredto as motion compensation.

Next, a specific example of a frame rate conversion method is described.With this method, frame rate conversion multiplied by a given rationalnumber (n/m) can be realized. Here, each of n and m is an integer equalto or more than 1. A frame rate conversion method in this embodimentmode can be treated as being divided into a first step and a secondstep. The first step is a step in which a frame rate is converted bybeing multiplied by the given rational number (n/m). As theinterpolation image, the basic image or the intermediate image obtainedby motion compensation may be used. The second step is a step in which aplurality of different images (sub-images) are generated from the imagedata to be input or from images each of which frame rate is converted inthe first step and the plurality of sub-images are continuouslydisplayed. By use of a method of the second step, human eyes can be madeto perceive display such that the display appears to be an originalimage, despite the fact that a plurality of different images aredisplayed.

Note that in the frame rate conversion method in this embodiment mode,both the first and second steps can be used, the second step only can beused with the first step omitted, or the first step only can be usedwith the second step omitted.

First, as the first step, frame rate conversion multiplied by the givenrational number (n/m) is described with reference to FIG. 70. In FIG.70, the horizontal axis represents time, and the vertical axisrepresents cases for various combinations of n and m. Each pattern inFIG. 70 is a schematic diagram of an image to be displayed, and ahorizontal position of the pattern represents timing of display. A dotin the pattern schematically represents movement of an image. Note thateach of these images is an example for explanation, and an image to bedisplayed is not limited to one of these images. This method can beapplied to a variety of images.

The period T_(in) represents a cycle of input image data. The cycle ofinput image data corresponds to an input frame rate. For example, whenthe input frame rate is 60 Hz, the cycle of input image data is 1/60seconds. Similarly, when the input frame rate is 50 Hz, the cycle ofinput image data is 1/50 seconds. Accordingly, the cycle (unit: second)of input image data is an inverse number of the input frame rate (unit:Hz). Note that a variety of input frame rates such as 24 Hz, 50 Hz, 60Hz, 70 Hz, 48 Hz, 100 Hz, 120 Hz, and 140 Hz can be used. 24 Hz is aframe rate for movies on film, for example. 50 Hz is a frame rate for avideo signal of the PAL standard, for example. 60 Hz is a frame rate fora video signal of the NTSC standard, for example. 70 Hz is a frame rateof a display input signal of a personal computer, for example. 48 Hz,100 Hz, 120 Hz, and 140 Hz are twice as high as 24 Hz, 50 Hz, 60 Hz, and70 Hz, respectively. Note that the frame rate can not only be doubledbut also multiplied by a variety of numbers. As described above, withthe method shown in this embodiment mode, a frame rate can be convertedwith respect to an input signal of various standards.

Procedures of frame rate conversion multiplied by the given rationalnumber (n/m) times in the first step are as follows. As a procedure 1,display timing of a k-th interpolation image (k is an integer equal toor more than 1, where the initial value is 1) with respect to a firstbasic image is decided. The display timing of the k-th interpolationimage is at the timing of passage of a period obtained by multiplicationof the cycle of input image data by k(m/n) after the first basic imageis displayed. As a procedure 2, whether the coefficient k(m/n) used fordeciding the display timing of the k-th interpolation image is aninteger or not is determined. When the coefficient k is an integer, a(k(m/n)+1)th basic image is displayed at the display timing of the k-thinterpolation image, and the first step is finished. When thecoefficient k is not an integer, the operation proceeds to a procedure3. As the procedure 3, an image used as the k-th interpolation image isdecided. Specifically, the coefficient k(m/n) used for deciding thedisplay timing of the k-th interpolation image is converted into theform (x+(y/n)). Each of x and y is an integer, and y is smaller than n.When an intermediate image obtained by motion compensation is employedas the k-th interpolation image, an intermediate image which is an imagecorresponding to movement obtained by multiplication of the amount ofmovement from an (x+1)th basic image to an (x+2)th basic image by (y/n)is employed as the k-th interpolation image. When the k-th interpolationimage is the same image as the basic image, the (x+1)th basic image canbe used. Note that a method for obtaining an intermediate image as animage corresponding to movement obtained by multiplication of the amountof movement of the image by (y/n) will be described in detail later. Asa procedure 4, a next interpolation image is set to be the objectiveinterpolation image. Specifically, the value of k is increased by one,and the operation returns to the procedure 1.

Next, the procedures in the first step are described in detail usingspecific values of n and m.

Note that a mechanism for performing the procedures in the first stepmay be mounted on a device or decided in the design phase of the devicein advance. When the mechanism for performing the procedures in thefirst step is mounted on the device, a driving method can be switched sothat optimal operations depending on circumstances can be performed.Note that the circumstances here include contents of image data,environment inside and outside the device (e.g., temperature, humidity,barometric pressure, light, sound, electric field, the amount ofradiation, altitude, acceleration, or movement speed), user settings,software version, and the like. On the other hand, when the mechanismfor performing the procedures in the first step is decided in the designphase of the device in advance, driver circuits optimal for respectivedriving methods can be used. Moreover, since the mechanism is decided,reducing manufacturing cost can be expected due to efficiency of massproduction.

When n=1 and m=1, that is, when a conversion ratio (n/m) is 1 (where n=1and m=1 in FIG. 70), an operation in the first step is as follows. Whenk=1, in the procedure 1, display timing of a first interpolation imagewith respect to the first basic image is decided. The display timing ofthe first interpolation image is at the timing of passage of a periodobtained by multiplication of the length of the cycle of input imagedata by k(m/n), that is, 1 after the first basic image is displayed.

Next, in the procedure 2, whether the coefficient k(m/n) used fordeciding the display timing of the first interpolation image is aninteger or not is determined Here, the coefficient k(m/n) is 1, which isan integer. Consequently, the (k(m/n)+1)th basic image, that is, asecond basic image is displayed at the display timing of the firstinterpolation image, and the first step is finished.

In other words, when the conversion ratio is 1, the k-th image is abasic image, the (k+1)th image is a basic image, and an image displaycycle is equal to the cycle of input image data.

Specifically, in a driving method of a display device in which, when theconversion ratio is 1 (n/m=1), i-th image data (1 is a positive integer)and (i+1)th image data are sequentially input as input image data in acertain cycle and the k-th image (k is a positive integer) and the(k+1)th image are sequentially displayed at an interval equal to thecycle of the input image data, the k-th image is displayed in accordancewith the i-th image data, and the (k+1)th image is displayed inaccordance with the (i+1)th image data.

Since the frame rate conversion circuit can be omitted when theconversion ratio is 1, manufacturing cost can be reduced. Further, whenthe conversion ratio is 1, quality of moving images can be improvedcompared with the case where the conversion ratio is less than 1Moreover, when the conversion ratio is 1, power consumption andmanufacturing cost can be reduced compared with the case where theconversion ratio is more than 1.

When n=2 and m=1, that is, when the conversion ratio (n/m) is 2 (wheren=2 and m=1 in FIG. 70), an operation in the first step is as follows.When k=1, in the procedure 1, display timing of the first interpolationimage with respect to the first basic image is decided. The displaytiming of the first interpolation image is at the timing of passage of aperiod obtained by multiplication of the length of the cycle of inputimage data by k(m n), that is, 1/2 after the first basic image isdisplayed.

Next, in the procedure 2, whether the coefficient k(m/n) used fordeciding the display timing of the first interpolation image is aninteger or not is determined. Here, the coefficient k(m/n) is 1/2, whichis not an integer. Consequently, the operation proceeds to the procedure3.

In the procedure 3, an image used as the first interpolation image isdecided. In order to decide the image, the coefficient 1/2 is convertedinto the form (x+(y/n)). In the case of the coefficient 1/2, x=0 andy=1. When an intermediate image obtained by motion compensation isemployed as the first interpolation image, an intermediate imagecorresponding to movement obtained by multiplication of the amount ofmovement from the (x+1)th basic image, that is, the first basic image tothe (x+2)th basic image, that is, the second basic image by (y/n), thatis, 1/2 is employed as the first interpolation image. When the firstinterpolation image is the same image as the basic image, the (x+1)thbasic image, that is, the first basic image can be used.

According to the procedures performed up to this point, the displaytiming of the first interpolation image and the image displayed as thefirst interpolation image can be decided. Next, in the procedure 4, theobjective interpolation image is shifted from the first interpolationimage to a second interpolation image. That is, k is changed from 1 to2, and the operation returns to the procedure 1.

When k=2, in the procedure 1, display timing of the second interpolationimage with respect to the first basic image is decided. The displaytiming of the second interpolation image is at the timing of passage ofa period obtained by multiplication of the length of the cycle of inputimage data by k(m/n), that is, 1 after the first basic image isdisplayed.

Next, in the procedure 2, whether the coefficient k(m/n) used fordeciding the display timing of the second interpolation image is aninteger or not is determined. Here, the coefficient k(m/n) is 1, whichis an integer. Consequently, the (k(m/n)+1)th basic image, that is, thesecond basic image is displayed at the display timing of the secondinterpolation image, and the first step is finished.

In other words, when the conversion ratio is 2 (n/m=2), the k-th imageis a basic image, the (k+1)th image is an interpolation image, a (k+2)thimage is a basic image, and an image display cycle is half the cycle ofinput image data.

Specifically, in a driving method of a display device in which, when theconversion ratio is 2 (n1 m=2), the i-th image data (i is a positiveinteger) and the (i+1)th image data are sequentially input as inputimage data in a certain cycle and the k-th image (k is a positiveinteger), the (k+1)th image, and the (k+2)th image are sequentiallydisplayed at an interval which is half the cycle of the input imagedata, the k-th image is displayed in accordance with the i-th imagedata, the (k+1)th image is displayed in accordance with image datacorresponding to movement obtained by multiplication of the amount ofmovement from the i-th image data to the (i+1)th image data by 1/2, andthe (k+2)th image is displayed in accordance with the (i+1)th imagedata.

Even specifically, in a driving method of a display device in which,when the conversion ratio is 2 (n/m=2), the i-th image data (i is apositive integer) and the (i+1)th image data are sequentially input asinput image data in a certain cycle and the k-th image (k is a positiveinteger), the (k+1)th image, and the (k+2)th image are sequentiallydisplayed at an interval which is half the cycle of the input imagedata, the k-th image is displayed in accordance with the i-th imagedata, the (k+1)th image is displayed in accordance with the i-th imagedata, and the (k+2)th image is displayed in accordance with the (i+1)thimage data.

Specifically, when the conversion ratio is 2, driving is also referredto as double-frame rate driving or frame rate driving. For example, whenthe input frame rate is 60 Hz, the display frame rate is 120 Hz (120 Hzdriving). Accordingly, two images are continuously displayed withrespect to one input image. At this time, when an interpolation image isan intermediate image obtained by motion compensation, the movement ofmoving images can be made to be smooth; thus, quality of the movingimage can be significantly improved. Further, quality of moving imagescan be significantly improved particularly when the display device is anactive matrix liquid crystal display device. This is related to aproblem of lack of writing voltage due to change in the electrostaticcapacity of a liquid crystal element by applied voltage, so-calleddynamic capacitance. That is, when the display frame rate is made higherthan the input frame rate, the frequency of a writing operation of imagedata can be increased; thus, defects such as an afterimage and aphenomenon of a moving image in which traces are seen due to lack ofwriting voltage because of dynamic capacitance can be reduced. Moreover,a combination of 120 Hz driving and alternating-current driving of aliquid crystal display device is effective. That is, when drivingfrequency of the liquid crystal display device is 120 Hz and frequencyof alternating-current driving is an integer multiple of 120 Hz or aunit fraction of 120 Hz (e.g., 30 Hz, 60 Hz, 120 Hz, or 240 Hz),flickers which appear in alternating-current driving can be reduced to alevel that cannot be perceived by human eyes.

When n=3 and m=1, that is, when the conversion ratio (n/m) is 3 (where nand m=1 in FIG. 70), an operation in the first step is as follows.First, when k=1, in the procedure 1, display timing of the firstinterpolation image with respect to the first basic image is decided.The display timing of the first interpolation image is at the timing ofpassage of a period obtained by multiplication of the length of thecycle of input image data by k(m/n), that is, 1/3 after the first basicimage is displayed.

Next, in the procedure 2, whether the coefficient k(m/n) used fordeciding the display timing of the first interpolation image is aninteger or not is determined. Here, the coefficient k(m/n) is 1/3, whichis not an integer. Consequently, the operation proceeds to the procedure3.

In the procedure 3, an image used as the first interpolation image isdecided. In order to decide the image, the coefficient 1/3 is convertedinto the form (x+(y/n)). In the case of the coefficient 1/3, x=0 andy=1. When an intermediate image obtained by motion compensation isemployed as the first interpolation image, an intermediate imagecorresponding to movement obtained by multiplication of the amount ofmovement from the (x+1)th basic image, that is, the first basic image tothe (x+2)th basic image, that is, the second basic image by (y/n), thatis, 1/3 is employed as the first interpolation image. When the firstinterpolation image is the same image as the basic image, the (x+1)thbasic image, that is, the first basic image can be used.

According to the procedures performed up to this point, the displaytiming of the first interpolation image and the image displayed as thefirst interpolation image can be decided. Next, in the procedure 4, theobjective interpolation image is shifted from the first interpolationimage to the second interpolation image. That is, k is changed from 1 to2, and the operation returns to the procedure 1.

When k=2, in the procedure 1, display timing of the second interpolationimage with respect to the first basic image is decided. The displaytiming of the second interpolation image is at the timing of passage ofa period obtained by multiplication of the length of the cycle of inputimage data by k(m/n), that is, 2/3 after the first basic image isdisplayed.

Next, in the procedure 2, whether the coefficient k(m/n) used fordeciding the display timing of the second interpolation image is aninteger or not is determined. Here, the coefficient k(m/n) is 2/3, whichis not an integer. Consequently, the operation proceeds to the procedure3.

In the procedure 3, an image used as the second interpolation image isdecided. In order to decide the image, the coefficient 2/3 is convertedinto the form (x+(y/n)). In the case of the coefficient 2/3, x=0 andy=2. When an intermediate image obtained by motion compensation isemployed as the second interpolation image, an intermediate imagecorresponding to movement obtained by multiplication of the amount ofmovement from the (x+1)th basic image, that is, the first basic image tothe (x+2)th basic image, that is, the second basic image by (y/n), thatis, 2/3 is employed as the second interpolation image. When the secondinterpolation image is the same image as the basic image, the (x+1)thbasic image, that is, the first basic image can be used.

According to the procedures performed up to this point, the displaytiming of the second interpolation image and the image displayed as thesecond interpolation image can be decided. Next, in the procedure 4, theobjective interpolation image is shifted from the second interpolationimage to a third interpolation image. That is, k is changed from 2 to 3,and the operation returns to the procedure 1.

When k=3, in the procedure 1, display timing of the third interpolationimage with respect to the first basic image is decided. The displaytiming of the third interpolation image is at the timing of passage of aperiod obtained by multiplication of the length of the cycle of inputimage data by k(m/n), that is, 1 after the first basic image isdisplayed.

Next, in the procedure 2, whether the coefficient k(m/n) used fordeciding the display timing of the third interpolation image is aninteger or not is determined. Here, the coefficient k(m n) is 1, whichis an integer. Consequently, the (k(m/n)+1)th basic image, that is, thesecond basic image is displayed at the display timing of the thirdinterpolation image, and the first step is finished.

In other words, when the conversion ratio is 3 (n/m=3), the k-th imageis a basic image, the (k+1)th image is an interpolation image, the(k+2)th image is an interpolation image, a (k+3)th image is a basicimage, and an image display cycle is 1/3 times the cycle of input imagedata.

Specifically, in a driving method of a display device in which, when theconversion ratio is 3 (n/m=3), the i-th image data (i is a positiveinteger) and the (i+1)th image data are sequentially input as inputimage data in a certain cycle and the k-th image (k is a positiveinteger), the (k+1)th image, the (k+2)th image, and the (k+3)th imageare sequentially displayed at an interval which is 1/3 times the cycleof the input image data, the k-th image is displayed in accordance withthe i-th image data, the (k+1)th image is displayed in accordance withimage data corresponding to movement obtained by multiplication of theamount of movement from the i-th image data to the (i+1)th image data by1/3, the (k+2)th image is displayed in accordance with image datacorresponding to movement obtained by multiplication of the amount ofmovement from the i-th image data to the (i+1)th image data by 2/3, andthe (k+3)th image is displayed in accordance with the (i+1)th imagedata.

Even specifically, in a driving method of a display device in which,when the conversion ratio is 3 (n1 m=3), the i-th image data (i is apositive integer) and the (i+1)th image data are sequentially input asinput image data in a certain cycle and the k-th image (k is a positiveinteger), the (k+1)th image, the (k+2)th image, and the (k+3)th imageare sequentially displayed at an interval which is 1/3 times the cycleof the input image data, the k-th image is displayed in accordance withthe i-th image data, the (k+1)th image is displayed in accordance withthe i-th image data, the (k+2)th image is displayed in accordance withthe i-th image data, and the (k+3)th image is displayed in accordancewith the (i+1)th image data.

When the conversion ratio is 3, quality of moving images can be improvedcompared with the case where the conversion ratio is less than 3.Moreover, when the conversion ratio is 3, power consumption andmanufacturing cost can be reduced compared with the case where theconversion ratio is more than 3.

Specifically, when the conversion ratio is 3, driving is also referredto as triple-frame rate driving. For example, when the input frame rateis 60 Hz, the display frame rate is 180 Hz (180 Hz driving).Accordingly, three images are continuously displayed with respect to oneinput image. At this time, when an interpolation image is anintermediate image obtained by motion compensation, the movement ofmoving images can be made to be smooth; thus, quality of the movingimage can be significantly improved. Further, when the display device isan active matrix liquid crystal display device, a problem of lack ofwriting voltage due to dynamic capacitance can be avoided; thus, qualityof moving images can be significantly improved, in particular withrespect to defects such as an afterimage and a phenomenon of a movingimage in which traces are seen. Moreover, a combination of 180 Hzdriving and alternating-current driving of a liquid crystal displaydevice is effective. That is, when driving frequency of the liquidcrystal display device is 180 Hz and frequency of alternating-currentdriving is an integer multiple of 180 Hz or a unit fraction of 180 Hz(e.g., 45 Hz, 90 Hz, 180 Hz, or 360 Hz), flickers which appear inalternating-current driving can be reduced to a level that cannot beperceived by human eyes.

When n=3 and m=2, that is, when the conversion ratio (n/m) is 3/2 (wheren=3 and m=2 in FIG. 70), an operation in the first step is as follows.When k=1, in the procedure 1, the display timing of the firstinterpolation image with respect to the first basic image is decided.The display timing of the first interpolation image is at the timing ofpassage of a period obtained by multiplication of the length of thecycle of input image data by km/n), that is, 2/3 after the first basicimage is displayed.

Next, in the procedure 2, whether the coefficient k(m/n) used fordeciding the display timing of the first interpolation image is aninteger or not is determined. Here, the coefficient k(m/n) is 2/3, whichis not an integer. Consequently, the operation proceeds to the procedure3.

In the procedure 3, an image used as the first interpolation image isdecided. In order to decide the image, the coefficient 2/3 is convertedinto the form (x+(y/n)). In the case of the coefficient 2/3, x=0 andy=2. When an intermediate image obtained by motion compensation isemployed as the first interpolation image, an intermediate imagecorresponding to movement obtained by multiplication of the amount ofmovement from the (x+1)th basic image, that is, the first basic image tothe (x+2)th basic image, that is, the second basic image by (y/n), thatis, 2/3 is employed as the first interpolation image. When the firstinterpolation image is the same image as the basic image, the (x+1)thbasic image, that is, the first basic image can be used.

According to the procedures performed up to this point, the displaytiming of the first interpolation image and the image displayed as thefirst interpolation image can be decided. Next, in the procedure 4, theobjective interpolation image is shifted from the first interpolationimage to the second interpolation image. That is, k is changed from 1 to2, and the operation returns to the procedure 1.

When k=2, in the procedure 1, the display timing of the secondinterpolation image with respect to the first basic image is decided.The display timing of the second interpolation image is at the timing ofpassage of a period obtained by multiplication of the length of thecycle of input image data by k(m/n), that is, 4/3 after the first basicimage is displayed.

Next, in the procedure 2, whether the coefficient k(m/n) used fordeciding the display timing of the second interpolation image is aninteger or not is determined. Here, the coefficient k(m/n) is 4/3, whichis not an integer. Consequently, the operation proceeds to the procedure3.

In the procedure 3, an image used as the second interpolation image isdecided. In order to decide the image, the coefficient 4/3 is convertedinto the form (x+(y/n)). In the case of the coefficient 4/3, x=1 andy=1. When an intermediate image obtained by motion compensation isemployed as the second interpolation image, an intermediate imagecorresponding to movement obtained by multiplication of the amount ofmovement from the (x+1)th basic image, that is, the second basic imageto the (x+2)th basic image, that is, a third basic image by (y/n), thatis, 1/3 is employed as the second interpolation image. When the secondinterpolation image is the same image as the basic image, the (x+1)thbasic image, that is, the second basic image can be used.

According to the procedures performed up to this point, the displaytiming of the second interpolation image and the image displayed as thesecond interpolation image can be decided. Next, in the procedure 4, theobjective interpolation image is shifted from the second interpolationimage to the third interpolation image. That is, k is changed from 2 to3, and the operation returns to the procedure 1.

When k=3, in the procedure 1, the display timing of the thirdinterpolation image with respect to the first basic image is decided.The display timing of the third interpolation image is at the timing ofpassage of a period obtained by multiplication of the length of thecycle of input image data by k(m/n), that is, 2 after the first basicimage is displayed.

Next, in the procedure 2, whether the coefficient k(m/n) used fordeciding the display timing of the third interpolation image is aninteger or not is determined. Here, the coefficient k(m/n) is 2, whichis an integer. Consequently, the (k(m/n)+1)th basic image, that is, thethird basic image is displayed at the display timing of the thirdinterpolation image, and the first step is finished.

In other words, when the conversion ratio is 3/2 (n/m=3/2), the k-thimage is a basic image, the (k+1)th image is an interpolation image, the(k+2)th image is an interpolation image, the (k+3)th image is a basicimage, and an image display cycle is 2/3 times the cycle of input imagedata.

Specifically, in a driving method of a display device in which, when theconversion ratio is 3/2 (n/m=3/2), the i-th image data (i is a positiveinteger), the (i+1)th image data, and (i+2)th image data aresequentially input as input image data in a certain cycle and the k-thimage (k is a positive integer), the (k+1)th image, the (k+2)th image,and the (k+3)th image are sequentially displayed at an interval which is2/3 times the cycle of the input image data, the k-th image is displayedin accordance with the i-th image data, the (k+1)th image is displayedin accordance with image data corresponding to movement obtained bymultiplication of the amount of movement from the i-th image data to the(i+1)th image data by 2/3, the (k+2)th image is displayed in accordancewith image data corresponding to movement obtained by multiplication ofthe amount of movement from the (i+1)th image data to the (i+2)th imagedata by 1/3, and the (k+3)th image is displayed in accordance with the(i+2)th image data.

Even specifically, in a driving method of a display device in which,when the conversion ratio is 3/2 (n/m=3/2), the i-th image data (i is apositive integer), the (i+1)th image data, and the (i+2)th image dataare sequentially input as input image data in a certain cycle and thek-th image (k is a positive integer), the (k+1)th image, the (k+2)thimage, and the (k+3)th image are sequentially displayed at an intervalwhich is 2/3 times the cycle of the input image data, the k-th image isdisplayed in accordance with the i-th image data, the (k+1)th image isdisplayed in accordance with the i-th image data, the (k+2)th image isdisplayed in accordance with the (i+1)th image data, and the (k+3)thimage is displayed in accordance with the (i+2)th image data.

When the conversion ratio is 3/2, quality of moving images can beimproved compared with the case where the conversion ratio is less than3/2. Moreover, when the conversion ratio is 3/2, power consumption andmanufacturing cost can be reduced compared with the case where theconversion ratio is more than 3/2.

Specifically, when the conversion ratio is 3/2, driving is also referredto as 3/2-fold frame rate driving or 1.5-fold frame rate driving. Forexample, when the input frame rate is 60 Hz, the display frame rate is90 Hz (90 Hz driving). Accordingly, three images are continuouslydisplayed with respect to two input images. At this time, when aninterpolation image is an intermediate image obtained by motioncompensation, the movement of moving images can be made to be smooth;thus, quality of the moving image can be significantly improved.Moreover, operating frequency of a circuit used for obtaining anintermediate image by motion compensation can be reduced, in particular,compared with a driving method with high driving frequency, such as 120Hz driving (double-frame rate driving) or 180 Hz driving (triple-framerate driving); thus, an inexpensive circuit can be used, andmanufacturing cost and power consumption can be reduced. Further, whenthe display device is an active matrix liquid crystal display device, aproblem of lack of writing voltage due to dynamic capacitance can beavoided; thus, quality of moving images can be significantly improved,in particular with respect to defects such as an afterimage and aphenomenon of a moving image in which traces are seen. Moreover, acombination of 90 Hz driving and alternating-current driving of a liquidcrystal display device is effective. That is, when driving frequency ofthe liquid crystal display device is 90 Hz and frequency ofalternating-current driving is an integer multiple of 90 Hz or a unitfraction of 90 Hz (e.g., 30 Hz, 45 Hz, 90 Hz, or 180 Hz), flickers whichappear in alternating-current driving can be reduced to a level thatcannot be perceived by human eyes.

Detailed description of procedures for positive integers n and m otherthan those described above is omitted. A conversion ratio can be set asa given rational number (n/m) in accordance with the procedures of framerate conversion in the first step. Note that among combinations of thepositive integers n and in, a combination in which a conversion ratio(n/m) can be reduced to its lowest term can be treated the same as aconversion ratio that is already reduced to its lowest term.

For example, when n=4 and m=1, that is, when the conversion ratio (n/m)is 4 (where n=4 and m=1 in FIG. 70), the k-th image is a basic image,the (k+1)th image is an interpolation image, the (k+2)th image is aninterpolation image, the (k+3)th image is an interpolation image, a(k+4)th image is a basic image, and an image display cycle is 1/4 timesthe cycle of input image data.

Specifically, in a driving method of a display device in which, when theconversion ratio is 4 (n/m=4), the i-th image data (i is a positiveinteger) and the (i+1)th image data are sequentially input as inputimage data in a certain cycle and the k-th image (k is a positiveinteger), the (k+1)th image, the (k+2)th image, the (k+3)th image, andthe (k+4)th image are sequentially displayed at an interval which is 1/4times the cycle of the input image data, the k-th image is displayed inaccordance with the i-th image data, the (k+1)th image is displayed inaccordance with image data corresponding to movement obtained bymultiplication of the amount of movement from the i-th image data to the(i+1)th image data by 1/4, the (k+2)th image is displayed in accordancewith image data corresponding to movement obtained by multiplication ofthe amount of movement from the i-th image data to the (i+1)th imagedata by 1/2, the (k+3)th image is displayed in accordance with imagedata corresponding to movement obtained by multiplication of the amountof movement from the i-th image data to the (i+1)th image data by 3/4,and the (k+4)th image is displayed in accordance with the (i+1)th imagedata.

Even specifically, in a driving method of a display device in which,when the conversion ratio is 4 (n/m=4), the i-th image data (i is apositive integer) and the (i+1)th image data are sequentially input asinput image data in a certain cycle and the k-th image (k is a positiveinteger), the (k+1)th image, the (k+2)th image, the (k+3)th image, andthe (k+4)th image are sequentially displayed at an interval which is 1/4times the cycle of the input image data, the k-th image is displayed inaccordance with the i-th image data, the (k+1)th image is displayed inaccordance with the i-th image data, the (k+2)th image is displayed inaccordance with the i-th image data, the (k+3)th image is displayed inaccordance with the i-th image data, and the (k+4)th image is displayedin accordance with the (i+1)th image data.

When the conversion ratio is 4, quality of moving images can be improvedcompared with the case where the conversion ratio is less than 4.Moreover, when the conversion ratio is 4, power consumption andmanufacturing cost can be reduced compared with the case where theconversion ratio is more than 4.

Specifically, when the conversion ratio is 4, driving is also referredto as quadruple-frame rate driving. For example, when the input framerate is 60 Hz, the display frame rate is 240 Hz (240 Hz driving).Accordingly, four images are continuously displayed with respect to oneinput image. At this time, when an interpolation image is anintermediate image obtained by motion compensation, the movement ofmoving images can be made to be smooth; thus, quality of the movingimage can be significantly improved. Moreover, an interpolation imageobtained by more accurate motion compensation can be used, inparticular, compared with a driving method with low driving frequency,such as 120 Hz driving (double-frame rate driving) or 180 Hz driving(triple-frame rate driving); thus, the movement of moving images can bemade smoother, and quality of the moving image can be significantlyimproved. Further, when the display device is an active matrix liquidcrystal display device, a problem of lack of writing voltage due todynamic capacitance can be avoided; thus, quality of moving images canbe significantly improved, in particular with respect to defects such asan afterimage and a phenomenon of a moving image in which traces areseen. Moreover, a combination of 240 Hz driving and alternating-currentdriving of a liquid crystal display device is effective. That is, whendriving frequency of the liquid crystal display device is 240 Hz andfrequency of alternating-current driving is an integer multiple of 240Hz or a unit fraction of 240 Hz (e.g., 30 Hz, 40 Hz, 60 Hz, or 120 Hz),flickers which appear in alternating-current driving can be reduced to alevel that cannot be perceived by human eyes.

Moreover, when n=4 and m=3, that is, when the conversion ratio (n/m) is4/3 (where n=4 and m=3 in FIG. 70), the k-th image is a basic image, the(k+1)th image is an interpolation image, the (k+2)th image is aninterpolation image, the (k+3)th image is an interpolation image, the(k+4)th image is a basic image, and the length of an image display cycleis 3/4 times the cycle of input image data.

As a further specific description, in a driving method of a displaydevice in which when the conversion ratio is 4/3 (n/m=4/3), the i-thimage data (i is a positive integer), the (i+1)th image data, the(i+2)th image data, and the (i+3)th image data are sequentially input asinput image data in a certain cycle and the k-th image (k is a positiveinteger), the (k+1)th image, the (k+2)th image, the (k+3)th image, andthe (k+4)th image are sequentially displayed at an interval which is 3/4times the cycle of the input image data, the k-th image is displayed inaccordance with the i-th image data, the (k+1)th image is displayed inaccordance with image data corresponding to movement obtained bymultiplying the amount of movement from the i-th image data to the(i+1)th image data by 3/4, the (k+2)th image is displayed in accordancewith image data corresponding to movement obtained by multiplying theamount of movement from the (i+1)th image data to the (i+2)th image databy 1/2, the (k+3)th image is displayed in accordance with image datacorresponding to movement obtained by multiplying the amount of movementfrom the (i+2)th image data to the (i+3)th image data by 1/4, and the(k+4)th image is displayed in accordance with the (i+3)th image data.

As a further specific description, in a driving method of a displaydevice in which when the conversion ratio is 4/3 (n/m=4/3), the i-thimage data (i is a positive integer), the (i+1)th image data, the(i+2)th image data, and the (i+3)th image data are sequentially input asinput image data in a certain cycle and the k-th image (k is a positiveinteger), the (k+1)th image, the (k+2)th image, the (k+3)th image, andthe (k+4)th image are sequentially displayed at an interval which is 3/4times the cycle of the input image data, the k-th image is displayed inaccordance with the i-th image data, the (k+1)th image is displayed inaccordance with the i-th image data, the (k+2)th image is displayed inaccordance with the (i+1)th image data, the (k+3)th image is displayedin accordance with the (i+2)th image data, and the (k+4)th image isdisplayed in accordance with the (i+3)th image data.

When the conversion ratio is 4/3, quality of moving images can beimproved compared with the case where the conversion ratio is less than4/3. Moreover, when the conversion ratio is 4/3, power consumption andmanufacturing cost can be reduced compared with the case where theconversion ratio is more than 4/3.

Specifically, when the conversion ratio is 4/3, driving is also referredto as 4/3-fold frame rate driving or 1.25-fold frame rate driving. Forexample, when the input frame rate is 60 Hz, the display frame rate is80 Hz (80 Hz driving). Four images are successively displayed withrespect to three input images. At this time, when an interpolation imageis an intermediate image obtained by motion compensation, motion ofmoving images can be made smooth; thus, quality of the moving image canbe significantly improved. Moreover, operating frequency of a circuitfor obtaining an intermediate image by motion compensation can bereduced particularly as compared with a driving method with high drivingfrequency, such as 120 Hz driving (double-frame rate driving) or 180 Hzdriving (triple-frame rate driving); thus, an inexpensive circuit can beused, and manufacturing cost and power consumption can be reduced.Further, when a display device is an active matrix liquid crystaldisplay device, a problem of shortage of writing voltage due to dynamiccapacitance can be avoided; thus, quality of moving images can besignificantly improved particularly with respect to defects such astraces and afterimages of a moving image. Moreover, a combination of 80Hz driving and alternating-current driving of a liquid crystal displaydevice is effective. That is, when driving frequency of the liquidcrystal display device is 80 Hz and frequency of alternating-currentdriving is an integer multiple of 80 Hz or a unit fraction of 80 Hz(e.g., 40 Hz, 80 Hz, 160 Hz, or 240 Hz), a flicker which appears byalternating-current driving can be reduced to the extent that theflicker is not perceived by human eyes.

Moreover, when n=5 and m=1, that is, when the conversion ratio (n/m) is5 (where n=5 and m=1 in FIG. 70), the k-th image is a basic image, the(k+1)th image is an interpolation image, the (k+2)th image is aninterpolation image, the (k+3)th image is an interpolation image, a(k+4)th image is an interpolation image, a (k+5)th image is a basicimage, and the length of an image display cycle is 1/5 times the cycleof input image data.

As a further specific description, in a driving method of a displaydevice in which when the conversion ratio is 5 (n/m=5), the i-th imagedata (i is a positive integer) and the (i+1)th image data aresequentially input as input image data in a certain cycle and the k-thimage (k is a positive integer), the (k+1)th image, the (k+2)th image,the (k+3)th image, the (k+4)th image, and the (k+5)th image aresequentially displayed at an interval whose length is 1/5 times thecycle of the input image data, the k-th image is displayed in accordancewith the i-th image data, the (k+1)th image is displayed in accordancewith image data corresponding to movement obtained by multiplying theamount of movement from the i-th image data to the (i+1)th image data by1/5, the (k+2)th image is displayed in accordance with image datacorresponding to movement obtained by multiplying the amount of movementfrom the i-th image data to the (i+1)th image data by 2/5, the (k+3)thimage is displayed in accordance with image data corresponding tomovement obtained by multiplying the amount of movement from the i-thimage data to the (i+1)th image data by 3/5, the (k+4)th image isdisplayed in accordance with image data corresponding to movementobtained by multiplying the amount of movement from the i-th image datato the (i+1)th image data by 4/5, and the (k+5)th image is displayed inaccordance with the (i+1)th image data.

As a further specific description, in a driving method of a displaydevice in which when the conversion ratio is 5 (n/m=5), the i-th imagedata (i is a positive integer) and the (i+1)th image data aresequentially input as input image data in a certain cycle and the k-thimage (k is a positive integer), the (k+1)th image, the (k+2)th image,the (k+3)th image, the (k+4)th image, and the (k+5)th image aresequentially displayed at an interval whose length is 1/5 times thecycle of the input image data, the k-th image is displayed in accordancewith the i-th image data, the (k+1)th image is displayed in accordancewith the i-th image data, the (k+2)th image is displayed in accordancewith the i-th image data, the (k+3)th image is displayed in accordancewith the i-th image data, the (k+4)th image is displayed in accordancewith the i-th image data, and the (k+5)th image is displayed inaccordance with the (i+1)th image data.

When the conversion ratio is 5, quality of moving images can be improvedcompared with the case where the conversion ratio is less than 5.Moreover, when the conversion ratio is 5, power consumption andmanufacturing cost can be reduced compared with the case where theconversion ratio is more than 5.

Specifically, when the conversion ratio is 5, driving is also referredto as 5-fold frame rate driving. For example, when the input frame rateis 60 Hz, the display frame rate is 300 Hz (300 Hz driving). Five imagesare successively displayed with respect to one input image. At thistime, when an interpolation image is an intermediate image obtained bymotion compensation, motion of moving images can be made smooth; thus,quality of the moving image can be significantly improved. Moreover, anintermediate image obtained by more accurate motion compensation can beused as the interpolation image particularly as compared with a drivingmethod with low driving frequency, such as 120 Hz driving (double-framerate driving) or 180 Hz driving (triple-frame rate driving); thus,motion of moving images can be made smoother, and quality of the movingimage can be significantly improved. Further, when a display device isan active matrix liquid crystal display device, a problem of shortage ofwriting voltage due to dynamic capacitance can be avoided; thus, qualityof moving images can be significantly improved particularly with respectto defects such as traces and afterimages of a moving image. Moreover, acombination of 300 Hz driving and alternating-current driving of aliquid crystal display device is effective. That is, when drivingfrequency of the liquid crystal display device is 300 Hz and frequencyof alternating-current driving is an integer multiple of 300 Hz or aunit fraction of 300 Hz (e.g., 30 Hz, 50 Hz, 60 Hz, or 100 Hz), aflicker which appears by alternating-current driving can be reduced tothe extent that the flicker is not perceived by human eyes.

Moreover, when n=5 and m=2, that is, when the conversion ratio (n/m) is5/2 (where n=5 and m=2 in FIG. 70), the k-th image is a basic image, the(k+1)th image is an interpolation image, the (k+2)th image is aninterpolation image, the (k+3)th image is an interpolation image, a(k+4)th image is an interpolation image, the (k+5)th image is a basicimage, and the length of an image display cycle is 2/5 times the cycleof input image data.

As a further specific description, in a driving method of a displaydevice in which when the conversion ratio is 5/2 (n/m=5/2), the i-thimage data (i is a positive integer), the (i+1)th image data, and the(i+2)th image data are sequentially input as input image data in acertain cycle and the k-th image (k is a positive integer), the (k+1)thimage, the (k+2)th image, the (k+3)th image, the (k+4)th image, and the(k+5)th image are sequentially displayed at an interval whose length is2/5 times the cycle of the input image data, the k-th image is displayedin accordance with the i-th image data, the (k+1)th image is displayedin accordance with image data corresponding to movement obtained bymultiplying the amount of movement from the i-th image data to the(i+1)th image data by 2/5, the (k+2)th image is displayed in accordancewith image data corresponding to movement obtained by multiplying theamount of movement from the i-th image data to the (i+1)th image data by4/5, the (k+3)th image is displayed in accordance with image datacorresponding to movement obtained by multiplying the amount of movementfrom the (i+1)th image data to the (i+2)th image data by 1/5, the(k+4)th image is displayed in accordance with image data correspondingto movement obtained by multiplying the amount of movement from the(i+1)th image data to the (i+2)th image data by 3/5, and the (k+5)thimage is displayed in accordance with the (i+2)th image data.

As a further specific description, in a driving method of a displaydevice in which when the conversion ratio is 5/2 (n1 m=5/2), the i-thimage data (i is a positive integer), the (I+1)th image data, the(i+2)th image data, and the (i+3)th image data are sequentially input asinput image data in a certain cycle and the k-th image (k is a positiveinteger), the (k+1)th image, the (k+2)th image, the (k+3)th image, the(k+4)th image, and the (k+5)th image are sequentially displayed at aninterval whose length is 2/5 times the cycle of the input image data,the k-th image is displayed in accordance with the i-th image data, the(k+1)th image is displayed in accordance with the i-th image data, the(k+2)th image is displayed in accordance with the i-th image data, the(k+3)th image is displayed in accordance with the (i+1)th image data,the (k 4)th image is displayed in accordance with the (i+1)th imagedata, and the (k+5)th image is displayed in accordance with the (i+2)thimage data.

When the conversion ratio is 5/2, quality of moving images can beimproved compared with the case where the conversion ratio is less than5/2. Moreover, when the conversion ratio is 5/2, power consumption andmanufacturing cost can be reduced compared with the case where theconversion ratio is more than 5/2.

Specifically, when the conversion ratio is 5/2, driving is also referredto as 5/2-fold frame rate driving or 2.5-fold frame rate driving. Forexample, when the input frame rate is 60 Hz, the display frame rate is150 Hz (150 Hz driving). Five images are successively displayed withrespect to two input images. At this time, when an interpolation imageis an intermediate image obtained by motion compensation, motion ofmoving images can be made smooth; thus, quality of the moving image canbe significantly improved. Moreover, an intermediate image obtained bymore accurate motion compensation can be used as the interpolation imageparticularly as compared with a driving method with low drivingfrequency, such as 120 Hz driving (double-frame rate driving); thus,motion of moving images can be made smoother, and quality of the movingimage can be significantly improved. Further, operating frequency of acircuit for obtaining an intermediate image by motion compensation canbe reduced particularly as compared with a driving method with highdriving frequency, such as 180 Hz driving (triple-frame rate driving);thus, an inexpensive circuit can be used, and manufacturing cost andpower consumption can be reduced. Furthermore, when a display device isan active matrix liquid crystal display device, a problem of shortage ofwriting voltage due to dynamic capacitance can be avoided; thus, qualityof moving images can be significantly improved particularly with respectto defects such as traces and afterimages of a moving image. Moreover, acombination of 150 Hz driving and alternating-current driving of aliquid crystal display device is effective. That is, when drivingfrequency of the liquid crystal display device is 150 Hz and frequencyof alternating-current driving is an integer multiple of 150 Hz or aunit fraction of 150 Hz (e.g., 30 Hz, 50 Hz, 75 Hz, or 150 Hz), aflicker which appears by alternating-current driving can be reduced tothe extent that the flicker is not perceived by human eyes.

In this manner, by setting positive integers n and m to be variousnumbers, the conversion ratio can be set to be a given rational number(n/m), Although detailed description is omitted, when n is 10 or less,combinations listed below can be possible: n=1, m=1, that is, theconversion ratio is (n/m)=1 (one-fold frame rate driving, 60 Hz), n=2,m=1, that is, the conversion ratio is (n/m)=2 (double-frame ratedriving, 120 Hz), n=3, m=1, that is, the conversion ratio is (n/m)=3(triple-frame rate driving, 180 Hz), 71=3, m=2, that is, the conversionratio is (n/m)=3/2 (3/2-fold frame rate driving, 90 Hz), n=4, m=1, thatis, the conversion ratio is (n/m)=4 (quadruple-frame rate driving, 240Hz), n=4, m=3, that is, the conversion ratio is (n/m)=4/3 (4/3-foldframe rate driving, 80 Hz), n=5, m=1, that is, the conversion ratio is(n/m)=5/1 (5-fold frame rate driving, 300 Hz), n=5, m=2, that is, theconversion ratio is (n/m)=5/2 (5/2-fold frame rate driving, 150 Hz),n=5, m=3, that is, the conversion ratio is (n/m)=5/3 (5/3-fold framerate driving, 100 Hz), n=5, m=4, that is, the conversion ratio is(n/m)=5/4 (5/4-fold frame rate driving, 75 Hz), n=6, m=1, that is, theconversion ratio is (n/m)=6 (6-fold frame rate driving, 360 Hz), n=6,m=5, that is, the conversion ratio is (n/m)=6/5 (6/5-fold frame ratedriving, 72 Hz), n=7, m=1, that is, the conversion ratio is (n/m)=7(7-fold frame rate driving, 420 Hz), n=7, m=2, that is; the conversionratio is (n/m)=7/2 (7/2-fold frame rate driving, 210 Hz), n=7, m=3, thatis, the conversion ratio is (n/m)=7/3 (7/3-fold frame rate driving, 140Hz), n=7, m=4, that is, the conversion ratio is (n/m)=7/4 (7/4-foldframe rate driving, 105 Hz), =7, m=5, that is, the conversion ratio is(n/m)=7/5 (7/5-fold frame rate driving, 84 Hz), n=7, m=6, that is, theconversion ratio is (n/m)=7/6 (7/6-fold frame rate driving, 70 Hz), n=8,m=1, that is, the conversion ratio is (n/m)=8 (8-fold frame ratedriving, 480 Hz), n=8, m=3, that is, the conversion ratio is (n/m)=8/3(8/3-fold frame rate driving, 160 Hz), n=8, m=5, that is, the conversionratio is (n/m)=8/5 (8/5-fold frame rate driving, 96 Hz), n=8, m=7, thatis, the conversion ratio is (n/m)=8/7 (8/7-fold frame rate driving, 68.6Hz), n=9, m=1, that is, the conversion ratio is (n/m)=9 (9-fold framerate driving, 540 Hz), n=9, m=2, that is, the conversion ratio is(n/m)=9/2 (9/2-fold frame rate driving, 270 Hz), n=9, m=4, that is, theconversion ratio is (n/m)=9/4 (9/4-fold frame rate driving, 135 Hz),n=9, m=5, that is, the conversion ratio is (n/m)=9/5 (9/5-fold framerate driving, 108 Hz), n=9, m=7, that is, the conversion ratio is(n/m)=9/7 (9/7-fold frame rate driving, 77.1 Hz), n=9, m=8, that is, theconversion ratio is (n/m)=9/8 (9/8-fold frame rate driving, 67.5 Hz),n=10, m=1, that is, the conversion ratio is (n/m)=10 (10-fold frame ratedriving, 600 Hz), n=10, m=3, that is, the conversion ratio is (n/m)=10/3(10/3-fold frame rate driving, 200 Hz), n=10, m=7, that is, theconversion ratio is (n/m)=10/7 (10/7-fold frame rate driving, 85.7 Hz),and n=10, m=9, that is, the conversion ratio is (n/m)=10/9 (10/9-foldframe rate driving, 66.7 Hz). Note that these frequencies are examplesin the case where the input frame rate is 60 Hz. With regard to otherframe rates, a product obtained by multiplication of each conversionratio and an input frame rate can be a driving frequency.

In the case where n is an integer more than 10, although specificnumbers for n and in are not stated here, the procedure of frame rateconversion in the first step can be obviously applied to various n andm.

Depending on how many images which can be displayed without motioncompensation to the input image data are included in the displayedimages, the conversion ratio can be determined. Specifically, thesmaller m becomes, the higher the proportion of images which can bedisplayed without motion compensation to the input image data becomes.When motion compensation is performed less frequently, power consumptioncan be reduced because a circuit which performs motion compensationoperates less frequently. In addition, the likelihood of generation ofan image (an intermediate image which does not correctly reflect motionof an image) including an error by motion compensation can be decreased,so that image quality can be improved. For example, as such a conversionratio, in the case where n is 10 or less, 1, 2, 3, 3/2, 4, 5, 5/2, 6, 7,7/2, 8, 9, 9/2, or 10 is possible. By employing such a conversion ratio,especially when an intermediate image obtained by motion compensation isused as an interpolation image, the image quality can be improved andpower consumption can be reduced because the number (half the totalnumber of images input) of images, which can be displayed without motioncompensation to the input image data, is comparatively large and motioncompensation is performed less frequently in the case where m is 2; andbecause the number (equal to the total number of images input) of imageswhich can be displayed without motion compensation to the input imagedata is large and motion compensation cannot be performed in the casewhere m is 1. On the other hand, the larger m becomes, the smoothermotion of images can be made because an intermediate image which isgenerated by motion compensation with high accuracy is used.

Note that, in the case where a display device is a liquid crystaldisplay device, the conversion ratio can be determined in accordancewith a response time of a liquid crystal element. Here, the responsetime of the liquid crystal element is the time from when a voltageapplied to the liquid crystal element is changed until when the liquidcrystal element responds. When the response time of the liquid crystalelement differs depending on the amount of change of the voltage appliedto the liquid crystal element, an average of the response times ofplural typical voltage changes can be used. Alternatively, the responsetime of the liquid crystal element can be defined as MRPT (movingpicture response time). Then, by frame rate conversion, the conversionratio which enables the length of the image display cycle to be near theresponse time of the liquid crystal element can be determined.Specifically, the response time of the liquid crystal element ispreferably the time from the value obtained by multiplication of thecycle of input image data and the inverse number of the conversionratio, to approximately half that value. In this manner, the imagedisplay cycle can be made to correspond to the response time of theliquid crystal element, so that the image quality is improved. Forexample, when the response time of the liquid crystal element is morethan or equal to 4 milliseconds and less than or equal to 8milliseconds, double-frame rate driving (120 Hz driving) can beemployed. This is because the image display cycle of 120 Hz driving isapproximately 8 milliseconds and the half of the image display cycle of120 Hz driving is approximately 4 milliseconds. Similarly, for example,when the response time of the liquid crystal element is more than orequal to 3 milliseconds and less than or equal to 6 milliseconds,triple-frame rate driving (180 Hz driving) can be employed; when theresponse time of the liquid crystal element is more than or equal to 5milliseconds and less than or equal to 11 milliseconds, 1.5-fold framerate driving (90 Hz driving) can be employed; when the response time ofthe liquid crystal element is more than or equal to 2 milliseconds andless than or equal to 4 milliseconds, quadruple-frame rate driving (240Hz driving) can be employed; and when the response time of the liquidcrystal element is more than or equal to 6 milliseconds and less than orequal to 12 milliseconds, 1.25-fold frame rate driving (80 Hz driving)can be employed. Note that this is similar to the case of other drivingfrequencies.

Note that the conversion ratio can also be determined by a tradeoffbetween the quality of the moving image, and power consumption andmanufacturing cost. That is, the quality of the moving image can beimproved by increasing the conversion ratio while power consumption andmanufacturing cost can be reduced by decreasing the conversion ratio.Therefore, when n is 10 or less, each conversion ratio has an advantagedescribed below.

When the conversion ratio is 1, the quality of the moving image can beimproved compared to the case where the conversion ratio is less than 1,and power consumption and manufacturing cost can be more reducedcompared to the case where the conversion ratio is more than 1.Moreover, since m is small, power consumption can be reduced while highimage quality is obtained. Further, by applying the conversion ratio of1 to a liquid crystal display device in which the response time of theliquid crystal elements is approximately 1 times the cycle of inputimage data, the image quality can be improved.

When the conversion ratio is 2, the quality of the moving image can bemore improved compared to the case where the conversion ratio is lessthan 2, and power consumption and manufacturing cost can be more reducedcompared to the case where the conversion ratio is more than 2.Moreover, since m is small, power consumption can be reduced while highimage quality is obtained. Further, by applying the conversion ratio of2 to a liquid crystal display device in which the response time of theliquid crystal elements is approximately 1/2 times the cycle of inputimage data, the image quality can be improved.

When the conversion ratio is 3, the quality of the moving image can bemore improved compared to the case where the conversion ratio is lessthan 3, and power consumption and manufacturing cost can be more reducedcompared to the case where the conversion ratio is more than 3.Moreover, since m is small, power consumption can be reduced while highimage quality is obtained. Further, by applying the conversion ratio of3 to a liquid crystal display device in which the response time of theliquid crystal elements is approximately 1/3 times the cycle of inputimage data, the image quality can be improved.

When the conversion ratio is 3/2, the quality of the moving image can bemore improved compared to the case where the conversion ratio is lessthan 3/2, and power consumption and manufacturing cost can be morereduced compared to the case where the conversion ratio is more than3/2. Moreover, since m is small, power consumption can be reduced whilehigh image quality is obtained. Further, by applying the conversionratio of 3/2 to a liquid crystal display device in which the responsetime of the liquid crystal elements is approximately 2/3 times the cycleof input image data, the image quality can be improved.

When the conversion ratio is 4, the quality of the moving image can bemore improved compared to the case where the conversion ratio is lessthan 4, and power consumption and manufacturing cost can be more reducedcompared to the case where the conversion ratio is more than 4.Moreover, since m is small, power consumption can be reduced while highimage quality is obtained. Further, by applying the conversion ratio of4 to a liquid crystal display device in which the response time of theliquid crystal elements is approximately 1/4 times the cycle of inputimage data, the image quality can be improved.

When the conversion ratio is 4/3, the quality of the moving image can bemore improved compared to the case where the conversion ratio is lessthan 4/3, and power consumption and manufacturing cost can be morereduced compared to the case where the conversion ratio is more than4/3. Moreover, since m is large, motion of the image can be madesmoother. Further, by applying the conversion ratio of 4/3 to a liquidcrystal display device in which the response time of the liquid crystalelements is approximately 3/4 times the cycle of input image data, theimage quality can be improved.

When the conversion ratio is 5, the quality of the moving image can bemore improved compared to the case where the conversion ratio is lessthan 5, and power consumption and manufacturing cost can be more reducedcompared to the case where the conversion ratio is more than 5.Moreover, since m is small, power consumption can be reduced while highimage quality is obtained. Further, by applying the conversion ratio of5 to a liquid crystal display device in which the response time of theliquid crystal elements is approximately 1/5 times the cycle of inputimage data, the image quality can be improved.

When the conversion ratio is 5/2, the quality of the moving image can bemore improved compared to the case where the conversion ratio is lessthan 5/2, and power consumption and manufacturing cost can be morereduced compared to the case where the conversion ratio is more than5/2. Moreover, since m is small, power consumption can be reduced whilehigh image quality is obtained. Further, by applying the conversionratio of 5/2 to a liquid crystal display device in which the responsetime of the liquid crystal elements is approximately 2/5 times the cycleof input image data, the image quality can be improved.

When the conversion ratio is 5/3, the quality of the moving image can bemore improved compared to the case where the conversion ratio is lessthan 5/3, and power consumption and manufacturing cost can be morereduced compared to the case where the conversion ratio is more than5/3. Moreover, since m is large, motion of the image can be madesmoother. Further, by applying the conversion ratio of 5/3 to a liquidcrystal display device in which the response time of the liquid crystalelements is approximately 3/5 times the cycle of input image data, theimage quality can be improved.

When the conversion ratio is 5/4, the quality of the moving image can bemore improved compared to the case where the conversion ratio is lessthan 5/4, and power consumption and manufacturing cost can be morereduced compared to the case where the conversion ratio is more than5/4. Moreover, since m is large, motion of the image can be madesmoother. Further, by applying the conversion ratio of 5/4 to a liquidcrystal display device in which the response time of the liquid crystalelements is approximately 4/5 times the cycle of input image data, theimage quality can be improved.

When the conversion ratio is 6, the quality of the moving image can bemore improved compared to the case where the conversion ratio is lessthan 6, and power consumption and manufacturing cost can be more reducedcompared to the case where the conversion ratio is more than 6.Moreover, since m is small, power consumption can be reduced while highimage quality is obtained. Further, by applying the conversion ratio of6 to a liquid crystal display device in which the response time of theliquid crystal elements is approximately 1/6 times the cycle of inputimage data, the image quality can be improved.

When the conversion ratio is 6/5, the quality of the moving image can bemore improved compared to the case where the conversion ratio is lessthan 6/5, and power consumption and manufacturing cost can be morereduced compared to the case where the conversion ratio is more than6/5. Moreover, since in is large, motion of the image can be madesmoother. Further, by applying the conversion ratio of 6/5 to a liquidcrystal display device in which the response time of the liquid crystalelements is approximately 5/6 times the cycle of input image data, theimage quality can be improved.

When the conversion ratio is 7, the quality of the moving image can bemore improved compared to the case where the conversion ratio is lessthan 7, and power consumption and manufacturing cost can be more reducedcompared to the case where the conversion ratio is more than 7.Moreover, since m is small, power consumption can be reduced while highimage quality is obtained. Further, by applying the conversion ratio of7 to a liquid crystal display device in which the response time of theliquid crystal elements is approximately 1/7 times the cycle of inputimage data, the image quality can be improved.

When the conversion ratio is 7/2, the quality of the moving image can bemore improved compared to the case where the conversion ratio is lessthan 7/2, and power consumption and manufacturing cost can be morereduced compared to the case where the conversion ratio is more than7/2. Moreover, since in is small, power consumption can be reduced whilehigh image quality is obtained. Further, by applying the conversionratio of 7/2 to a liquid crystal display device in which the responsetime of the liquid crystal elements is approximately 2/7 times the cycleof input image data, the image quality can be improved.

When the conversion ratio is 7/3, the quality of the moving image can bemore improved compared to the case where the conversion ratio is lessthan 7/3, and power consumption and manufacturing cost can be morereduced compared to the case where the conversion ratio is more than7/3. Moreover, since m is large, motion of the image can be madesmoother. Further, by applying the conversion ratio of 7/3 to a liquidcrystal display device in which the response time of the liquid crystalelements is approximately 3/7 times the cycle of input image data, theimage quality can be improved.

When the conversion ratio is 7/4, the quality of the moving image can bemore improved compared to the case where the conversion ratio is lessthan 7/4, and power consumption and manufacturing cost can be morereduced compared to the case where the conversion ratio is more than7/4. Moreover, since m is large, motion of the image can be madesmoother. Further, by applying the conversion ratio of 7/4 to a liquidcrystal display device in which the response time of the liquid crystalelements is approximately 4/7 times the cycle of input image data, theimage quality can be improved.

When the conversion ratio is 7/5, the quality of the moving image can bemore improved compared to the case where the conversion ratio is lessthan 7/5, and power consumption and manufacturing cost can be morereduced compared to the case where the conversion ratio is more than7/5. Moreover, since m is large, motion of the image can be madesmoother. Further, by applying the conversion ratio of 7/5 to a liquidcrystal display device in which the response time of the liquid crystalelements is approximately 5/7 times the cycle of input image data, theimage quality can be improved.

When the conversion ratio is 7/6, the quality of the moving image can bemore improved compared to the case where the conversion ratio is lessthan 7/6, and power consumption and manufacturing cost can be morereduced compared to the case where the conversion ratio is more than7/6. Moreover, since m is large, motion of the image can be madesmoother. Further, by applying the conversion ratio of 7/6 to a liquidcrystal display device in which the response time of the liquid crystalelements is approximately 6/7 times the cycle of input image data, theimage quality can be improved.

When the conversion ratio is 8, the quality of the moving image can bemore improved compared to the case where the conversion ratio is lessthan 8, and power consumption and manufacturing cost can be more reducedcompared to the case where the conversion ratio is more than 8.Moreover, since m is small, power consumption can be reduced while highimage quality is obtained. Further, by applying the conversion ratio of8 to a liquid crystal display device in which the response time of theliquid crystal elements is approximately 1/8 times the cycle of inputimage data, the image quality can be improved.

When the conversion ratio is 8/3, the quality of the moving image can bemore improved compared to the case where the conversion ratio is lessthan 8/3, and power consumption and manufacturing cost can be morereduced compared to the case where the conversion ratio is more than8/3. Moreover, since m is large, motion of the image can be madesmoother. Further, by applying the conversion ratio of 8/3 to a liquidcrystal display device in which the response time of the liquid crystalelements is approximately 3/8 times the cycle of input image data, theimage quality can be improved.

When the conversion ratio is 8/5, the quality of the moving image can bemore improved compared to the case where the conversion ratio is lessthan 8/5, and power consumption and manufacturing cost can be morereduced compared to the case where the conversion ratio is more than8/5. Moreover, since m is large, motion of the image can be madesmoother. Further, by applying the conversion ratio of 8/5 to a liquidcrystal display device in which the response time of the liquid crystalelements is approximately 5/8 times the cycle of input image data, theimage quality can be improved.

When the conversion ratio is 8/7, the quality of the moving image can bemore improved compared to the case where the conversion ratio is lessthan 8/7, and power consumption and manufacturing cost can be morereduced compared to the case where the conversion ratio is more than8/7. Moreover, since m is large, motion of the image can be madesmoother. Further, by applying the conversion ratio of 8/7 to a liquidcrystal display device in which the response time of the liquid crystalelements is approximately 7/8 times the cycle of input image data, theimage quality can be improved.

When the conversion ratio is 9, the quality of the moving image can bemore improved compared to the case where the conversion ratio is lessthan 9, and power consumption and manufacturing cost can be more reducedcompared to the case where the conversion ratio is more than 9.Moreover, since m is small, power consumption can be reduced while highimage quality is obtained. Further, by applying the conversion ratio of9 to a liquid crystal display device in which the response time of theliquid crystal elements is approximately 1/9 times the cycle of inputimage data, the image quality can be improved.

When the conversion ratio is 9/2, the quality of the moving image can bemore improved compared to the case where the conversion ratio is lessthan 9/2, and power consumption and manufacturing cost can be morereduced compared to the case where the conversion ratio is more than9/2. Moreover, since m is small, power consumption can be reduced whilehigh image quality is obtained. Further, by applying the conversionratio of 9/2 to a liquid crystal display device in which the responsetime of the liquid crystal elements is approximately 2/9 times the cycleof input image data, the image quality can be improved.

When the conversion ratio is 9/4, the quality of the moving image can bemore improved compared to the case where the conversion ratio is lessthan 9/4, and power consumption and manufacturing cost can be morereduced compared to the case where the conversion ratio is more than9/4. Moreover, since in is large, motion of the image can be madesmoother. Further, by applying the conversion ratio of 9/4 to a liquidcrystal display device in which the response time of the liquid crystalelements is approximately 4/9 times the cycle of input image data, theimage quality can be improved.

When the conversion ratio is 915, the quality of the moving image can bemore improved compared to the case where the conversion ratio is lessthan 9/5, and power consumption and manufacturing cost can be morereduced compared to the case where the conversion ratio is more than9/5. Moreover, since m is large, motion of the image can be madesmoother. Further, by applying the conversion ratio of 9/5 to a liquidcrystal display device in which the response time of the liquid crystalelements is approximately 5/9 times the cycle of input image data, theimage quality can be improved.

When the conversion ratio is 9/7, the quality of the moving image can bemore improved compared to the case where the conversion ratio is lessthan 9/7, and power consumption and manufacturing cost can be morereduced compared to the case where the conversion ratio is more than9/7. Moreover, since m is large, motion of the image can be madesmoother. Further, by applying the conversion ratio of 9/7 to a liquidcrystal display device in which the response time of the liquid crystalelements is approximately 7/9 times the cycle of input image data, theimage quality can be improved.

When the conversion ratio is 9/8, the quality of the moving image can bemore improved compared to the case where the conversion ratio is lessthan 9/8, and power consumption and manufacturing cost can be morereduced compared to the case where the conversion ratio is more than9/8. Moreover, since m is large, motion of the image can be madesmoother. Further, by applying the conversion ratio of 9/8 to a liquidcrystal display device in which the response time of the liquid crystalelements is approximately 8/9 times the cycle of input image data, theimage quality can be improved.

When the conversion ratio is 10, the quality of the moving image can bemore improved compared to the case where the conversion ratio is lessthan 10, and power consumption and manufacturing cost can be morereduced compared to the case where the conversion ratio is more than 10.Moreover, since m is small, power consumption can be reduced while highimage quality is obtained. Further, by applying the conversion ratio of10 to a liquid crystal display device in which the response time of theliquid crystal elements is approximately 1/10 times the cycle of inputimage data, the image quality can be improved.

When the conversion ratio is 10/3, the quality of the moving image canbe more improved compared to the case where the conversion ratio is lessthan 10/3, and power consumption and manufacturing cost can be morereduced compared to the case where the conversion ratio is more than10/3. Moreover, since m is large, motion of the image can be madesmoother. Further, by applying the conversion ratio of 10/3 to a liquidcrystal display device in which the response time of the liquid crystalelements is approximately 3/10 times the cycle of input image data, theimage quality can be improved.

When the conversion ratio is 10/7, the quality of the moving image canbe more improved compared to the case where the conversion ratio is lessthan 10/7, and power consumption and manufacturing cost can be morereduced compared to the case where the conversion ratio is more than10/7. Moreover, since m is large, motion of the image can be madesmoother. Further, by applying the conversion ratio of 10/7 to a liquidcrystal display device in which the response time of the liquid crystalelements is approximately 7/10 times the cycle of input image data, theimage quality can be improved.

When the conversion ratio is 10/9, the quality of the moving image canbe more improved compared to the case where the conversion ratio is lessthan 10/9, and power consumption and manufacturing cost can be morereduced compared to the case where the conversion ratio is more than10/9. Moreover, since m is large, motion of the image can be madesmoother. Further, by applying the conversion ratio of 10/9 to a liquidcrystal display device in which the response time of the liquid crystalelements is approximately 9/10 times the cycle of input image data, theimage quality can be improved.

Note that it is obvious that each conversion ratio where n is more than10 also has a similar advantage.

Next, as the second step, a method will be described in which aplurality of different images (sub-images) are generated from an imagebased on input image data or each image (hereinafter referred to as anoriginal image) whose frame rate is converted by a given rational number(n/m) times in the first step, and the plurality of sub-images aredisplayed in temporal succession. In this manner, a method of the secondstep can make human eyes perceive as if one original image weredisplayed in appearance, despite the fact that a plurality of differentimages are displayed.

Here, among the sub-images generated from one original image, asub-image which is displayed first is referred to as a first sub-image.The timing when the first sub-image is displayed is the same as thetiming when the original image determined in the first step isdisplayed. On the other hand, a sub-image which is displayed after thatis referred to as a second sub-image. The timing when the secondsub-image is displayed can be determined at will regardless of thetiming when the original image determined in the first step isdisplayed. Note that an image which is actually displayed is an imagegenerated from the original image by a method in the second step.Various images can be used for the original image for generatingsub-images. The number of sub-images is not limited to two and more thantwo sub-images are also possible. In the second step, the number ofsub-images is represented as J (J is an integer of 2 or more). At thattime, a sub-image which is displayed at the same timing as the timingwhen the original image determined in the first step is displayed isreferred to as a first sub-image. Sub-images which are sequentiallydisplayed are referred to as a second sub-image, a third sub image . . .and J-th sub-image in order from a sub-image which is displayed.

There are many methods for generating a plurality of sub-images from oneoriginal image. As main ones, the following methods can be given. Thefirst one is a method in which the original image is used as it is asthe sub-image. The second one is a method in which brightness of theoriginal image is distributed to the plurality of sub-images. The thirdone is a method in which an intermediate image obtained by motioncompensation is used as the sub-image.

Here, a method for distributing brightness of the original image to theplurality of sub-images can be further divided into some methods. Asmain ones, the following methods can be given. The first one is a methodin which at least one sub-image is a black image (hereinafter referredto as black data insertion). The second one is a method in which thebrightness of the original image is distributed to a plurality of rangesand just one sub-image among all the sub-images is used to control thebrightness in the ranges (hereinafter referred to as time-division grayscale control). The third one is a method in which one sub-image is abright image which is made by changing a gamma value of the originalimage, and the other sub-image is a dark image which is made by changingthe gamma value of the original image (hereinafter referred to as gammacomplement).

Some of the methods described above will be briefly described. In themethod in which the original image is used as it is as the sub-image,the original image is used as it is as the first sub-image. Further, theoriginal image is used as it is as the second sub-image. By using thismethod, a circuit which newly generates a sub-image does not need tooperate, or the circuit itself is not necessary, so that powerconsumption and manufacturing cost can be reduced. Particularly in aliquid crystal display device, this method is preferably used afterframe rate conversion using an intermediate image obtained by motioncompensation in the first step as an interpolation image. This isbecause defects such as traces and afterimages of a moving imageattributed to shortage of writing voltage due to dynamic capacitance ofthe liquid crystal elements can be reduced by using the intermediateimage obtained by motion compensation as the interpolation image to makemotion of the moving image smooth and displaying the same imagerepeatedly.

Next, in the method in which the brightness of the original image isdistributed to the plurality of sub-images, a method for setting thebrightness of the image and the length of a period when the sub-imagesare displayed will be specifically described. Note that J is the numberof sub-images, and an integer of 2 or more. The lower case j and capitalJ are distinguished. The lower case j is an integer of more than orequal to 1 and less than or equal to J. The brightness of a pixel innormal hold driving is L, the cycle of original image data is T, thebrightness of a pixel in a j-th sub-image is L_(j), and the length of aperiod when the j-th sub-image is displayed is T_(j). The total sum ofproducts of L_(j) and T_(j) where j=1 to where j=J (L₁T₁+L₂T₂+ . . .+L_(J)T_(J)) is preferably equal to a product of L and T (LT)(brightness is unchangeable). Further, the total sum of T_(j) where j=1to where j=J is preferably equal to T (a display cycle of the originalimage is maintained). Here, unchangeableness of brightness andmaintenance of the display cycle of the original image is referred to assub-image distribution condition.

In the methods for distributing brightness of the original image to aplurality of sub-images, black data insertion is a method in which atleast one sub-image is made a black image. In this manner, a displaymethod can be made close to pseudo impulse type display so thatdeterioration of quality of moving image due to hold-type display methodcan be prevented. In order to prevent a decrease in brightness due toblack data insertion, sub-image distribution condition is preferablysatisfied. However, in the situation that a decrease in brightness ofthe displayed image is acceptable (dark surrounding or the like) or inthe case where a decrease in brightness of the displayed image is set tobe acceptable by the user, sub-image distribution condition is notnecessarily satisfied. For example, one sub-image may be the same as theoriginal image and the other sub-image can be a black image. In thiscase, power consumption can be reduced compared to the case wheresub-image distribution condition is satisfied. Further, in a liquidcrystal display device, when one sub-image is made by increasing thewhole brightness of the original image without limitation of the maximumbrightness, sub-image distribution condition can be satisfied byincreasing brightness of a backlight. In this case, since sub-imagedistribution condition can be satisfied without controlling the voltagevalue which is applied to a pixel, operation of an image processingcircuit can be omitted, so that power consumption can be reduced.

Note that a feature of black data insertion is to make L_(j) of allpixels 0 in any one of sub-images. In this manner, a display method canbe made close to pseudo-impulse type display, so that deterioration ofquality of a moving image due to a hold-type display method can beprevented.

In the methods for distributing the brightness of the original image toa plurality of sub-images, time-division gray scale control is a methodin which brightness of the original image is divided into a plurality ofranges and brightness in that range is controlled by just one sub-imageamong all sub-images. In this manner, a display method can be made closeto pseudo impulse type display without a decrease in brightness.Therefore, deterioration of quality of moving image due to a hold-typedisplay method can be prevented.

As a method for dividing the brightness of the original image into aplurality of ranges, a method in which the maximum brightness (L_(max))is divided into the number of sub-images can be given. This method willbe described with a display device which can adjust brightness of 0 toL_(max) by 256 grades (from the grade 0 to 255) in the case where twosub-images are provided. When the grade 0 to 127 is displayed,brightness of one sub-image is adjusted in a range of the grade 0 to 255while brightness of the other sub-image is set to be the grade 0. Whenthe grade 128 to 255 is displayed, the brightness of on sub-image is setto be 255 while brightness of the other sub-image is adjusted in a rangeof the grade 0 to 255. In this manner, this method can make human eyesperceive as if an original image is displayed and make a display methodclose to pseudo-impulse type display, so that deterioration of qualityof an moving image due to a hold-type display method can be prevented.Note that more than two sub-images can be provided. For example, ifthree sub-images are provided, the grade (grade 0 to 255) of brightnessof an original image is divided into three. In some cases, the number ofgrades of brightness is not divisible by the number of sub-images,depending on the number of grades of brightness of the original imageand the number of sub-images; however, the number of grades ofbrightness which is included in a range of each divided brightness canbe distributed as appropriate even if the number of grades of brightnessis not just the same as the number of sub-images.

In the case of time-division gray scale control, by satisfying sub-imagedistribution condition, the same image as the original image can bedisplayed without a decrease in brightness or the like, which ispreferable.

In the methods for distributing brightness of the original image to aplurality of sub-images, gamma complement is a method in which onesub-image is made a bright image by changing the gamma characteristic ofthe original image while the other sub-image is made a dark image bychanging the gamma characteristic of the original image. In this manner,a display method can be made close to pseudo impulse type displaywithout a decrease in brightness. Therefore, deterioration of quality ofmoving image due to a hold-type display method can be prevented. Here, agamma characteristic is a degree of brightness with respect to a grade(gray scale) of brightness. In general, a line of the gammacharacteristic is adjusted so as to be close to a linear shape. This isbecause a smooth gray scale can be obtained if change in brightness isproportion to one gray scale in the grade of brightness. In gammacomplement, the curve of the gamma characteristic of one sub-image isdeviated from the linear shape so that the one sub-image is brighterthan a sub-image in the linear shape in a region of intermediatebrightness (halftone) (the image in halftone is brighter than as itusually is). Further, a line of the gamma characteristic of the othersub-image is also deviated from the linear shape so that the othersub-image is darker than the sub-image in the linear shape in a regionof intermediate brightness (the image in halftone is darker than as itusually is). Here, the amount of change for brightening the onesub-image than that in the linear shape, and the amount of change fordarkening the other sub-image than the sub-image in the linear shape,are preferably almost the same. This method can make human eyes perceiveas if an original image is displayed and a decrease in quality of amoving image due to a hold-type display method can be prevented. Notethat more than two sub-images can be provided. For example, if threesub-images are provided, each gamma characteristic of three sub-imagesare adjusted and the sum of the amounts of change for brighteningsub-images, and the sum of the amounts of change for darkeningsub-images are almost the same.

Note that also in the case of gamma complement, by satisfying sub-imagedistribution condition, the same image as the original image can bedisplayed without a decrease in brightness or the like, which ispreferable. Further, in gamma complement, since change in brightnessL_(j) of each sub-image with respect to gray scale follows a gammacurve, the gray scale of each sub-image can be displayed smoothly byitself. Therefore, there is an advantage that image quality to beperceived by human eyes is improved.

A method in which an intermediate image obtained by motion compensationis used as a sub-image is a method in which one sub-image is anintermediate image obtained by motion compensation using previous andnext images. In this manner, motion of images can be smooth and qualityof a moving image can be improved.

The relation between the timing when a sub-image is displayed and amethod of making a sub-image will be described. Although the timing whenthe first sub-image is displayed is the same as that when the originalimage determined in the first step is displayed, and the timing when thesecond sub-image is displayed can be decided at will regardless of thetiming when the original image determined in the first step isdisplayed, the sub-image itself may be changed in accordance with thetiming when the second sub-image is displayed. In this manner, even ifthe timing when the second sub-image is displayed is changed variously,human eyes can be made to perceive as if the original image isdisplayed. Specifically, if the timing when the second sub-image isdisplayed is earlier, the first sub-image can be brighter and the secondsub-image can be darker. Further, if the timing when the secondsub-image is displayed is later, the first sub-image may be darker andthe second sub-image may be brighter. This is because brightnessperceived by human eyes changes in accordance with the length of aperiod when an image is displayed. More specifically, the longer thelength of the period when an image is displayed becomes, the higherbrightness perceived by human eyes becomes while the shorter the lengthof the period when an image is displayed becomes, the lower brightnessperceived by human eyes becomes. That is, by making the timing when thesecond sub-image is displayed earlier, the length of the period when thefirst sub-image is displayed becomes shorter and the length of periodwhen the second sub-image is displayed becomes longer. This means humaneyes perceive as if the first sub-image is dark and the second sub-imageis bright. As a result, a different image from the original image isperceived by human eyes. In order to prevent this, the first sub-imagecan be made much brighter and the second sub-image can be made muchdarker. Similarly, by making the timing when the second sub-image isdisplayed later, the length of the period when the first sub-image isdisplayed becomes longer, and the length of the period when the secondsub-image is displayed becomes shorter; in such a case, the firstsub-image can be made much darker and the second sub-image can be mademuch brighter.

In accordance with the above description, procedures in the second stepis shown below. As a procedure 1, a method for making a plurality ofsub-images from one original image is decided. More specifically, amethod for making a plurality of sub-images can be selected from amethod in which an original image is used as it is as a sub-image, amethod in which brightness of an original image is distributed to aplurality of sub-images, and a method in which an intermediate imageobtained by motion compensation is used as a sub-image. As a procedure2, the number J of sub-images is decided. Note that J is an integer of 2or more. As a procedure 3, the brightness L_(j) of a pixel in j-thsub-image and the length of the period T_(j) when the j-th sub-image isdisplayed are decided in accordance with the method shown in theprocedure 1. Through the procedure 3, the length of a period when eachsub-image is displayed and the brightness of each pixel included in eachsub-image are specifically decided. As a procedure 4, the original imageis processed in accordance with what decided in respective procedures 1to 3 to actually perform display. As a procedure 5, the objectiveoriginal image is shifted to the next original image and the operationreturns to the procedure 1.

Note that a mechanism for performing the procedures in the second stepmay be mounted on a device or decided in the design phase of the devicein advance. When the mechanism for performing the procedures in thesecond step is mounted on the device, a driving method can be switchedso that an optimal operation depending on circumstances can beperformed. Note that the circumstances here include contents of imagedata, environment inside and outside the device (e.g., temperature,humidity, barometric pressure, light, sound, an electromagnetic field,an electric field, radiation quantity, an altitude, acceleration, ormovement speed), user setting, a software version, and the like. On theother hand, when the mechanism for performing the procedures in thesecond step is decided in the design phase of the device in advance,driver circuits optimal for respective driving methods can be used.Further, since the mechanism is decided, manufacturing cost can bereduced due to efficiency of mass production.

Next, various driving methods are employed depending on the proceduresin the second step and are described in detail, specifically showingvalues of n and m in the first step.

In the procedure 1 in the second step, in the case where a method usingan original image as it is as a sub-image is selected, the drivingmethod is as follows.

One feature of a driving method of the display device is that i-th (i isa positive integer) image data and (i+1)th image data are sequentiallyprepared in a constant cycle T. The cycle T is divided into J (J is aninteger equal to or more than 2) sub-image display periods. The i-thimage data is data which can make each of a plurality of pixels haveunique brightness L. The j-th (j is an integer equal to or more than 1,and equal to or less than J) sub-image is formed by arranging theplurality of pixels each having unique brightness L_(j), and is an imagedisplayed only during the j-th sub-image display period T_(j). Theaforementioned L, T, L_(j), and T_(j) satisfy the sub-image distributioncondition. In all values of j, the brightness L_(j) of each pixel whichis included in the j-th sub-image is equal to L. Here, as image datawhich are prepared sequentially in a constant cycle T, the originalimage data which is formed in the first step can be used. That is, alldisplay patterns given in the description of the first step can becombined with the above mentioned driving method.

Then, in the case where the number of sub-images J is determined to be 2in the procedure 2 in the second step, and it is determined thatT₁=T₂=T/2 in the procedure 3, the above-mentioned driving method is asshown in FIG. 71. In FIG. 71, the horizontal axis indicates time, andthe vertical axis indicates cases which are classified with respect tovarious values of n and m used in the first step.

For example, in the first step, in the case of n=1 and m=1, in otherwords, when the conversion ratio (n/m) is 1, a driving method as shownin the case of n=1 and m=1 in FIG. 71 is employed. At this time, thedisplay frame rate is twice (double-frame rate driving) as high as theframe rate of input image data. Specifically, for example, when theinput frame rate is 60 Hz, the display frame rate is 120 Hz (120 Hzdriving). Then, two images are continuously displayed with respect toone piece of input image data. Here, in the case of double-frame ratedriving, quality of moving images can be improved than the case wherethe frame rate is lower than that of the double-frame rate driving, andpower consumption and a production cost can be reduced than the casewhere the frame rate is higher than that of the double-frame ratedriving. Further, in the procedure 1 in the second step, when a methodin which an original image is used as it is as a sub-image is selected,a circuit operation which produces an intermediate image by motioncompensation can be stopped, or the circuit itself can be omitted fromthe device, whereby power consumption and a production cost of thedevice can be reduced. Further, when a display device is an activematrix liquid crystal display device, a problem of shortage of writingvoltage due to dynamic capacitance can be avoided; thus, quality ofmoving images can be significantly improved while defects, inparticular, such as a phenomenon of a moving image in which traces areseen and an afterimage are reduced. Moreover, a combination of 120 Hzdriving and alternating-current driving of a liquid crystal displaydevice is effective. That is, when the driving frequency of the liquidcrystal display device is 120 Hz and the frequency ofalternating-current driving is an integer multiple of 120 Hz or a unitfraction of 120 Hz (e.g., 30 Hz, 60 Hz, 120 Hz, or 240 Hz), flickerswhich appear by alternating-current driving can be reduced so as not tobe perceived by human eyes. Moreover, image quality can be improved byapplying the driving method to the liquid crystal display device inwhich the response time of the liquid crystal element is approximatelyhalf a cycle of input image data.

Further, for example, in the first step, in the case of n=2 and m=1, inother words, when the conversion ratio (n/m) is 2, a driving method asshown in the case of n=2 and m=1 in FIG. 71 is employed. At this time,the display frame rate is 4-fold (quadruple-frame rate driving) as highas the frame rate of input image data. Specifically, for example, whenthe input frame rate is 60 Hz, the display frame rate is 240 Hz (240 Hzdriving). Then, four images are continuously displayed with respect toone piece of input image data. At this time, when an interpolated imagein the first step is an intermediate image obtained by motioncompensation, motion of moving images can be smooth; thus, quality ofmoving images can be significantly improved. In the case ofquadruple-frame rate driving, quality of moving images can be improvedthan the case where the frame rate is lower than that of thequadruple-frame rate driving, and power consumption and a productioncost can be reduced than the case where the frame rate is higher thanthat of the quadruple-frame rate driving. Further, in the procedure 1 inthe second step, when a method in which an original image is used as itis as a sub-image is selected, a circuit operation which produces anintermediate image by motion compensation can be stopped, or the circuititself can be omitted from the device, whereby power consumption and aproduction cost of the device can be reduced. Further, when a displaydevice is an active matrix liquid crystal display device, a problem ofshortage of writing voltage due to dynamic capacitance can be avoided;thus, quality of moving images can be significantly improved whiledefects, in particularly, such as a phenomenon of a moving image inwhich traces are seen and an afterimage are reduced. Moreover, acombination of 240 Hz driving and alternating-current driving of aliquid crystal display device is effective. That is, when the drivingfrequency of the liquid crystal display device is 240 Hz and thefrequency of alternating-current driving is an integer multiple of 240Hz or a unit fraction of 240 Hz (e.g., 30 Hz, 60 Hz, 120 Hz, or 240 Hz),flickers which appear by alternating-current driving can be reduced soas not to be perceived by human eyes. Moreover, image quality can beimproved by applying the driving method to the liquid crystal displaydevice in which the response time of the liquid crystal element isapproximately quarter of a cycle of input image data.

Further, for example, in the first step, in the case of n=3 and m=1, inother words, when the conversion ratio (n/m) is 3, a driving method asshown in the case of n=3 and m=1 in FIG. 71 is employed. At this time,the display frame rate is 6-fold (6-fold frame rate driving) as high asthe frame rate of input image data. Specifically, for example, when theinput frame rate is 60 Hz, the display frame rate is 360 Hz (360 Hzdriving). Then, six images are continuously displayed with respect toone piece of input image data. At this time, when an interpolated imagein the first step is an intermediate image obtained by motioncompensation, motion of moving images can be smooth; thus, quality ofmoving images can be significantly improved. In the case of 6-fold framerate driving, quality of moving images can be improved than the casewhere the frame rate is lower than that of the 6-fold frame ratedriving, and power consumption and a production cost can be reduced thanthe case where the frame rate is higher than that of the 6-fold framerate driving. Further, in the procedure 1 in the second step, when amethod in which an original image is used as it is as a sub-image isselected, a circuit operation which produces an intermediate image bymotion compensation can be stopped, or the circuit itself can be omittedfrom the device, whereby power consumption and a production cost of thedevice can be reduced. Further, when a display device is an activematrix liquid crystal display device, a problem of shortage of writingvoltage due to dynamic capacitance can be avoided; thus, quality ofmoving images can be significantly improved while defects, inparticular, such as a phenomenon of a moving image in which traces areseen and an afterimage are reduced. Moreover, a combination of 360 Hzdriving and alternating-current driving of a liquid crystal displaydevice is effective. That is, when the driving frequency of the liquidcrystal display device is 360 Hz and the frequency ofalternating-current driving is an integer multiple of 360 Hz or a unitfraction of 360 Hz (e.g., 30 Hz, 60 Hz, 120 Hz, or 180 Hz), flickerswhich appear by alternating-current driving can be reduced so as not tobe perceived by human eyes. Moreover, image quality can be improved byapplying the driving method to the liquid crystal display device inwhich the response time of the liquid crystal element is approximately1/6 of a cycle of input image data.

Further, for example, in the first step, in the case of n=3 and m=2, inother words, when the conversion ratio (n/m) is 3/2, a driving method asshown in the case of n=3 and m=2 in FIG. 71 is employed. At this time,the display frame rate is triple (triple frame rate driving) as high asthe frame rate of input image data. Specifically, for example, when theinput frame rate is 60 Hz, the display frame rate is 180 Hz (180 Hzdriving). Then, three images are continuously displayed with respect toone piece of input image data. At this time, when an interpolated imagein the first step is an intermediate image obtained by motioncompensation, motion of moving images can be smooth; thus, quality ofmoving images can be significantly improved. In the case of triple framerate driving, quality of moving images can be improved than the casewhere the frame rate is lower than that of the triple frame ratedriving, and power consumption and a production cost can be reduced thanthe case where the frame rate is higher than that of the triple framerate driving. Further, in the procedure 1 in the second step, when amethod in which an original image is used as it is as a sub-image isselected, a circuit operation which produces an intermediate image bymotion compensation can be stopped, or the circuit itself can be omittedfrom the device, whereby power consumption and a production cost of thedevice can be reduced. Further, when a display device is an activematrix liquid crystal display device, a problem of shortage of writingvoltage due to dynamic capacitance can be avoided; thus, quality ofmoving images can be significantly improved while defects, inparticular, such as a phenomenon of a moving image in which traces areseen and an afterimage are reduced. Moreover, a combination of 180 Hzdriving and alternating-current driving of a liquid crystal displaydevice is effective. That is, when the driving frequency of the liquidcrystal display device is 180 Hz and the frequency ofalternating-current driving is an integer multiple of 180 Hz or a unitfraction of 180 Hz (e.g., 30 Hz, 60 Hz, 120 Hz, or 180 Hz), flickerswhich appear by alternating-current driving can be reduced so as not tobe perceived by human eyes. Moreover, image quality can be improved byapplying the driving method to the liquid crystal display device inwhich the response time of the liquid crystal element is approximately1/3 of a cycle of input image data.

Further, for example, in the first step, in the case of n=4 and m=1, inother words, when the conversion ratio (n/m) is 4, a driving method asshown in the case of n=4 and m=1 in FIG. 71 is employed. At this time,the display frame rate is 8-fold (8-fold frame rate driving) as high asthe frame rate of input image data. Specifically, for example, when theinput frame rate is 60 Hz, the display frame rate is 480 Hz (480 Hzdriving). Then, eight images are continuously displayed with respect toone piece of input image data. At this time, when an interpolated imagein the first step is an intermediate image obtained by motioncompensation, motion of moving images can be smooth; thus, quality ofmoving images can be significantly improved. In the case of 8-fold framerate driving, quality of moving images can be improved than the casewhere the frame rate is lower than that of the 8-fold frame ratedriving, and power consumption and a production cost can be reduced thanthe case where the frame rate is higher than that of the 8-fold framerate driving. Further, in the procedure 1 in the second step, when amethod in which an original image is used as it is as a sub-image isselected, a circuit operation which produces an intermediate image bymotion compensation can be stopped, or the circuit itself can be omittedfrom the device, whereby power consumption and a production cost of thedevice can be reduced. Further, when a display device is an activematrix liquid crystal display device, a problem of shortage of writingvoltage due to dynamic capacitance can be avoided; thus, quality ofmoving images can be significantly improved while defects, inparticular, such as a phenomenon of a moving image in which traces areseen and an afterimage are reduced. Moreover, a combination of 480 Hzdriving and alternating-current driving of a liquid crystal displaydevice is effective. That is, when the driving frequency of the liquidcrystal display device is 480 Hz and the frequency ofalternating-current driving is an integer multiple of 480 Hz or a unitfraction of 480 Hz (e.g., 30 Hz, 60 Hz, 120 Hz, or 240 Hz), flickerswhich appear by alternating-current driving can be reduced so as not tobe perceived by human eyes. Moreover, image quality can be improved byapplying the driving method to the liquid crystal display device inwhich the response time of the liquid crystal element is approximately1/8 of a cycle of input image data.

Further, for example, in the first step, in the case of n=4 and m=3, inother words, when the conversion ratio (n/m) is 4/3, a driving method asshown in the case of n=4 and m=3 in FIG. 71 is employed. At this time,the display frame rate is 8/3 times (8/3-fold frame rate driving) ashigh as the frame rate of input image data. Specifically, for example,when the input frame rate is 60 Hz, the display frame rate is 160 Hz(160 Hz driving). Then, eight images are continuously displayed withrespect to three pieces of input image data. At this time, when aninterpolated image in the first step is an intermediate image obtainedby motion compensation, motion of moving images can be smooth; thus,quality of moving images can be significantly improved. In the case of8/3-fold frame rate driving, quality of moving images can be improvedthan the case where the frame rate is lower than that of the 8/3-foldframe rate driving, and power consumption and a production cost can bereduced than the case where the frame rate is higher than that of the8/3-fold frame rate driving. Further, in the procedure 1 in the secondstep, when a method in which an original image is used as it is as asub-image is selected, a circuit operation which produces anintermediate image by motion compensation can be stopped, or the circuititself can be omitted from the 1.5 device, whereby power consumption anda production cost of the device can be reduced. Further, when a displaydevice is an active matrix liquid crystal display device, a problem ofshortage of writing voltage due to dynamic capacitance can be avoided;thus, quality of moving images can be significantly improved whiledefects, in particular, such as a phenomenon of a moving image in whichtraces are seen and an afterimage are reduced. Moreover, a combinationof 160 Hz driving and alternating-current driving of a liquid crystaldisplay device is effective. That is, when the driving frequency of theliquid crystal display device is 160 Hz and the frequency ofalternating-current driving is an integer multiple of 160 Hz or a unitfraction of 160 Hz (e.g., 40 Hz, 80 Hz, 160 Hz, or 320 Hz), flickerswhich appear by alternating-current driving can be reduced so as not tobe perceived by human eyes. Moreover, image quality can be improved byapplying the driving method to the liquid crystal display device inwhich the response time of the liquid crystal element is approximately3/8 of a cycle of input image data.

Further, for example, in the first step, in the case of n=5 and m=1, inother words, when the conversion ratio (n/m) is 5, a driving method asshown in the case of n=5 and m=1 in FIG. 71 is employed. At this time,the display frame rate is 10-fold (10-fold frame rate driving) as highas the frame rate of input image data Specifically, for example, whenthe input frame rate is 60 Hz, the display frame rate is 600 Hz (600 Hzdriving). Then, ten images are continuously displayed with respect toone piece of input image data. At this time, when an interpolated imagein the first step is an intermediate image obtained by motioncompensation, motion of moving images can be smooth; thus, quality ofmoving images can be significantly improved. In the case of 10-foldframe rate driving, quality of moving images can be improved than thecase where the frame rate is lower than that of the 10-fold frame ratedriving, and power consumption and a production cost can be reduced thanthe case where the frame rate is higher than that of the 10-fold framerate driving. Further, in the procedure 1 in the second step, when amethod in which an original image is used as it is as a sub-image isselected, a circuit operation which produces an intermediate image bymotion compensation can be stopped, or the circuit itself can be omittedfrom the device, whereby power consumption and a production cost of thedevice can be reduced. Further, when a display device is an activematrix liquid crystal display device, a problem of shortage of writingvoltage due to dynamic capacitance can be avoided; thus, quality ofmoving images can be significantly improved while defects, inparticular, such as a phenomenon of a moving image in which traces areseen and an afterimage are reduced. Moreover, a combination of 600 Hzdriving and alternating-current driving of a liquid crystal displaydevice is effective. That is, when the driving frequency of the liquidcrystal display device is 600 Hz and the frequency ofalternating-current driving is an integer multiple of 600 Hz or a unitfraction of 600 Hz (e.g., 30 Hz, 60 Hz, 100 Hz, or 120 Hz), flickerswhich appear by alternating-current driving can be reduced so as not tobe perceived by human eyes. Moreover, image quality can be improved byapplying the driving method to the liquid crystal display device inwhich the response time of the liquid crystal element is approximately1/10 of a cycle of input image data.

Further, for example, in the first step, in the case of n=5 and m=2, inother words, when the conversion ratio (n/m) is 5/2, a driving method asshown in the case of n=5 and m=2 in FIG. 71 is employed. At this time,the display frame rate is 5-fold (5-fold frame rate driving) as high asthe frame rate of input image data. Specifically, for example, when theinput frame rate is 60 Hz, the display frame rate is 300 Hz (300 Hzdriving). Then, five images are continuously displayed with respect toone piece of input image data. At this time, when an interpolated imagein the first step is an intermediate image obtained by motioncompensation, motion of moving images can be smooth; thus, quality ofmoving images can be significantly improved. In the case of 5-fold framerate driving, quality of moving images can be improved than the casewhere the frame rate is lower than that of the 5-fold frame ratedriving, and power consumption and a production cost can be reduced thanthe case where the frame rate is higher than that of the 5-fold framerate driving. Further, in the procedure 1 in the second step, when amethod in which an original image is used as it is as a sub-image isselected, a circuit operation which produces an intermediate image bymotion compensation can be stopped, or the circuit itself can be omittedfrom the device, whereby power consumption and a production cost of thedevice can be reduced. Further, when a display device is an activematrix liquid crystal display device, a problem of shortage of writingvoltage due to dynamic capacitance can be avoided; thus, quality ofmoving images can be significantly improved while defects, inparticular, such as a phenomenon of a moving image in which traces areseen and an afterimage are reduced. Moreover, a combination of 300 Hzdriving and alternating-current driving of a liquid crystal displaydevice is effective. That is, when the driving frequency of the liquidcrystal display device is 300 Hz and the frequency ofalternating-current driving is an integer multiple of 300 Hz or a unitfraction of 300 Hz (e.g., 30 Hz, 50 Hz, 60 Hz, or 100 Hz), flickerswhich appear by alternating-current driving can be reduced so as not tobe perceived by human eyes. Moreover, image quality can be improved byapplying the driving method to the liquid crystal display device inwhich the response time of the liquid crystal element is approximately1/5 of a cycle of input image data.

As described above, when a method in which an original image is used asit is as a sub-image is selected the procedure 1 in the second step; thenumber of sub-images is determined to be 2 in the procedure 2 in thesecond step; when it is determined that T₁=T₂=T/2 in the procedure 3 inthe second step, the display frame rate can be double of the displayframe rate obtained by the frame rate conversion using a conversionratio determined by the values of n and m in the first step; thus,quality of moving images can be further improved. Further, the qualityof moving images can be improved than the case where a display framerate is lower than the display frame rate, and power consumption and aproduction cost can be reduced than the case where a display frame rateis higher than the display frame rate. Further, in the procedure 1 inthe second step, when a method in which an original image is used as itis as a sub-image is selected, a circuit operation which produces anintermediate image by motion compensation can be stopped, or the circuititself can be omitted from the device, whereby power consumption and aproduction cost of the device can be reduced. Further, when a displaydevice is an active matrix liquid crystal display device, a problem ofshortage of writing voltage due to dynamic capacitance can be avoided;thus, quality of moving images can be significantly improved whiledefects, in particular, such as a phenomenon of a moving image in whichtraces are seen and an afterimage are reduced. Furthermore, when thedriving frequency of the liquid crystal display device is made high andthe frequency of alternating-current driving is an integer multiple or aunit fraction, flickers which appear by alternating-current driving canbe reduced so as not to be perceived by human eyes. Moreover, imagequality can be improved by applying the driving method to the liquidcrystal display device in which the response time of the liquid crystalelement is approximately (1/(double the conversion ratio)) of a cycle ofinput image data.

Note that it is obvious that there are similar advantages in the case ofusing a conversion ratio than those described above, though detaileddescription is omitted. For example when n is 10 or less, the followingcombinations are possible in addition to the above mentioned cases:

-   n=5, m=3, that is, the conversion ratio (n/m)=5/3 (10/3-fold frame    rate driving, 200 Hz),-   n=5, m=4, that is, the conversion ratio (n/m)=5/4 (5/2-fold frame    rate driving, 150 Hz),-   n=6, m=1, that is, the conversion ratio (n/m)=6 (12-fold frame rate    driving, 720 Hz),-   n=6, m=5, that is, the conversion ratio (n/m)=6/5 (12/5-fold frame    rate driving, 144 Hz),-   n=7, m=1, that is, the conversion ratio (n/m)=7 (14-fold frame rate    driving, 840 Hz),-   n=7, m=2, that is, the conversion ratio (n/m)=7/2 (7-fold frame rate    driving, 420 Hz),-   n=7, m=3, that is, the conversion ratio (n/m)=7/3 (14/3-fold frame    rate driving, 280 Hz),-   n=7, m=4, that is, the conversion ratio (n/m)=7/4 (7/2-fold frame    rate driving, 210 Hz),-   n=7, m=5, that is, the conversion ratio (n/m)=7/5 (14/5-fold frame    rate driving, 168 Hz),-   n=7, m=6, that is, the conversion ratio (n/m)=7/6 (7/3-fold frame    rate driving, 140 Hz),-   n=8, m=1, that is, the conversion ratio (n/m)=8 (16-fold frame rate    driving, 960 Hz),-   n=8, m=3, that is, the conversion ratio (n/m)=8/3 (16/3-fold frame    rate driving, 320 Hz),-   n=8, m=5, that is, the conversion ratio (n/m)=8/5 (16/5-fold frame    rate driving, 192 Hz),-   n=8, m=7, that is, the conversion ratio (n/m)=8/7 (16/7-fold frame    rate driving, 137 Hz),-   n=9, m=1, that is, the conversion ratio (n/m)=9 (18-fold frame rate    driving, 1080 Hz),-   n=9, m=2, that is, the conversion ratio (n/m)=9/2 (9-fold frame rate    driving, 540 Hz),-   n=9, m=4, that is, the conversion ratio (n/m)=9/4 (9/2-fold frame    rate driving, 270 Hz),-   n=9, m=5, that is, the conversion ratio (n/m)=9/5 (18/5-fold frame    rate driving, 216 Hz),-   n=9, m=7, that is, the conversion ratio (n/m)=9/7 (18/7-fold frame    rate driving, 154 Hz),-   n=9, m=8, that is, the conversion ratio (n/m)=9/8 (9/4-fold frame    rate driving, 135 Hz),-   n=10, m=1, that is, the conversion ratio (n/m)=10 (20-fold frame    rate driving, 1200 Hz),-   n=10, m=3, that is, the conversion ratio (n/m)=10/3 (20/3-fold frame    rate driving, 400 Hz),-   n=10, m=7, that is, the conversion ratio (n/m)=10/7 (20/7-fold frame    rate driving, 171 Hz), and-   n=10, m=9, that is, the conversion ratio (n/m)=10/9 (20/9-fold frame    rate driving, 133 Hz). Note that these frequencies are examples in    the case where the input frame rate is 60 Hz. As for other frame    rates, the product of an input frame rate multiplied by double of    conversion ratio in each case is a driving frequency.

Although specific numbers for n and m in the case where n is an integermore than 10 are not stated here, the procedure in the second step canbe obviously applied to various values of n and m.

Note that in the case of J=2, it is particularly effective that theconversion ratio in the first step is larger than 2. This is becausewhen the number of sub-images is comparatively smaller like J=2 in thesecond step, the conversion ratio in the first step can be higher. Sucha conversion ratio includes 3, 4, 5, 5/2, 6, 7, 7/2, 7/3, 8, 8/3, 9,9/2, 9/4, 10, and 10/3, when n is equal to or less than 10. When displayframe rate after the first step is such a value, by setting the value ofJ at 3 or more balance between an advantage (e.g., reduction of powerconsumption and a production cost) by the number of sub-images in thesecond step being small and an advantage (e.g., increase of moving imagequality, reduction of flickers) by the final display frame rate beinghigh can be achieved.

Note that although the case where the number of sub-images J isdetermined to be 2 in the procedure 2 and it is determined thatT₁=T₂=T/2 in the procedure 3 has been described here, the presentinvention is not limited to this obviously.

For example, in the case where it is determined that T₁<T₂ in theprocedure 3 in the second step, the first sub-image can be brightenedand the second sub-image can be darkened. Further, in the case where itis determined that T₁>T₂ in the procedure 3 in the second step, thefirst sub-image can be darkened and the second sub-image can bebrightened. Thus, a display method can be pseudo impulse driving, whilethe original image can be perceived by human eyes; therefore, quality ofmoving images can be improved. Note that when a method in which anoriginal image is used as it is as a sub-image is selected in theprocedure 1 as the case of the above-mentioned driving method, thesub-image can be directly displayed without changing the brightness ofthe sub-image. This is because an image which is used as a sub-image isthe same in this case, and the original image can be displayed properlyregardless of display timing of the sub-image.

Further, it is obvious that the number of sub-images J may be anothervalue instead of 2 in the procedure 2. In this case, the display framerate can be J times as high as the display frame rate obtained by theframe rate conversion using a conversion ratio determined by the valuesof n and m in the first step; thus, quality of moving images can befurther improved. Further, the quality of moving images can be improvedthan the case where a display frame rate is lower than the display framerate, and power consumption and a production cost can be reduced thanthe case where a display frame rate is higher than the display framerate. Further, in the procedure 1 in the second step, when a method inwhich an original image is used as it is as a sub-image is selected, acircuit operation which produces an intermediate image by motioncompensation can be stopped, or the circuit itself can be omitted fromthe device, whereby power consumption and a production cost of thedevice can be reduced. Further, when a display device is an activematrix liquid crystal display device, a problem of shortage of writingvoltage due to dynamic capacitance can be avoided; thus, quality ofmoving images can be significantly improved while defects, inparticular, such as a phenomenon of a moving image in which traces areseen and an afterimage are reduced. Furthermore, when the drivingfrequency of the liquid crystal display device is made high and thefrequency of alternating-current driving is an integer multiple or aunit fraction, flickers which appear by alternating-current driving canbe reduced so as not to be perceived by human eyes. Moreover, imagequality can be improved by applying the driving method to the liquidcrystal display device in which the response time of the liquid crystalelement is approximately (1/(J times the conversion ratio)) of a cycleof input image data.

For example, in the case of J=3, particularly there is advantages thatthe quality of moving images can be improved compared to the case wherethe number of sub-images is smaller than 3, and that power consumptionand a production cost can be reduced compared to the case where thenumber of sub-images is larger than 3. Moreover, image quality can beimproved by applying the driving method to the liquid crystal displaydevice in which the response time of the liquid crystal element isapproximately (1/(three times the conversion ratio)) of a cycle of inputimage data.

For example, in the case of J=4, particularly there is advantages thatthe quality of moving images can be improved compared to the case wherethe number of sub-images is smaller than 4, and that power consumptionand a production cost can be reduced compared to the case where thenumber of sub-images is larger than 4. Moreover, image quality can beimproved by applying the driving method to the liquid crystal displaydevice in which the response time of the liquid crystal element isapproximately (1/(four times the conversion ratio)) of a cycle of inputimage data.

For example, in the case of J=5, particularly there is advantages thatthe quality of moving images can be improved compared to the case wherethe number of sub-images is smaller than 5, and that power consumptionand a production cost can be reduced compared to the case where thenumber of sub-images is larger than 5. Moreover, image quality can beimproved by applying the driving method to the liquid crystal displaydevice in which the response time of the liquid crystal element isapproximately (1/(five times the conversion ratio)) of a cycle of inputimage data.

Furthermore, there are similar advantages even in the case where thenumber of J is anything other than the above mentioned numbers.

Note that in the case of J=3 or more, the conversion ratio in the firststep can be various values. J=3 or more is effective particularly whenthe conversion ratio in the first step is relatively small (equal to orless than 2). This is because when the display frame rate after thefirst step is relatively lower, J can be larger in the second step. Sucha conversion ratio includes 1, 2, 3/2, 4/3, 5/3, 5/4, 6/5, 7/4, 7/5,7/6, 8/7, 9/5, 9/7, 9/8, 10/7, and 10/9 when n is equal to or less than10. FIG. 72 shows the case where the conversion ratio is 1, 2, 3/2, 4/3,5/3, and 5/4 among the above-described conversion ratios. As describedabove, when the display frame rate after the first step is a relativelysmall value, by setting the value of J at 3 or more balance between anadvantage (e.g., reduction of power consumption and a production cost)by the number of sub-images in the first step being small and anadvantage (e.g., increase of moving image quality, reduction offlickers) by the final display frame rate being high can be achieved.

Next, another example of the driving method determined by the procedurein the second step will be described.

In the procedure 1 in the second step, when black data insertion isselected among methods in which brightness of the original image isdistributed to a plurality of sub-images, the driving method is asfollows.

One feature of a driving method of the display device is that i-th (i isa positive integer) image data and (i+1)th image data are sequentiallyprepared in a constant cycle T. The cycle T is divided into J (J is aninteger equal to or more than 2) sub-image display periods. The i-thimage data is data which can make each of a plurality of pixels haveunique brightness L. The j-th is an integer equal to or more than 1, andequal to or less than J) sub-image is formed by arranging a plurality ofpixels each having unique brightness L_(j), and is an image which isdisplayed only during the j-th sub-image display period T_(j). Theaforementioned L, T, L_(j), and T_(j) satisfy the sub-image distributioncondition. In at least one value of j, the brightness L_(j) of allpixels which are included in the j-th sub-image is equal to 0. Here, asimage data which are prepared sequentially in a constant cycle T, theoriginal image data which is formed in the first step can be used. Thatis, all display patterns given in the description of the first step canbe combined with the above mentioned driving method.

It is obvious that the driving method can be implemented by combiningvarious values of n and m which are used in the first step.

Then, when the number of sub-images J is determined to be 2 in theprocedure 2 in the second step, and it is determined that T₁=T₂=T/2 inthe procedure 3, the driving method can be as shown in FIG. 71. Sincefeatures and advantages of the driving method (display timing usingvarious values of n and m) shown in FIG. 71 have already been described,detailed description is omitted here. In the procedure 1 in the secondstep, even when black data insertion is selected among methods in whichbrightness of the original image is distributed to a plurality ofsub-images, it is obvious that similar advantages can be gained. Forexample, when an interpolated image in the first step is an intermediateimage obtained by motion compensation, motion of a moving image can besmooth; thus, quality of moving images can be significantly improved.The quality of moving images can be improved when the display frame rateis high, and power consumption and a production cost can be reduced whenthe display frame rate is low. Further, when a display device is anactive matrix liquid crystal display device, a problem of shortage ofwriting voltage due to dynamic capacitance can be avoided; thus, qualityof moving images can be significantly improved while defects, inparticular, such as a phenomenon of a moving image in which traces areseen and an afterimage are reduced. Flickers which appear byalternating-current driving can be reduced so as not to be perceived byhuman eyes.

In the procedure 1 in the second step, as a typical advantage ofselecting black data insertion among methods in which brightness of theoriginal image is distributed to a plurality of sub-images, a circuitoperation which produces an intermediate image by motion compensationcan be stopped, or the circuit itself can be omitted from the device,whereby power consumption and a production cost of the device can bereduced. Further, the display method can be pseudo impulse drivingregardless of the gray scale value included in the image data;therefore, quality of a moving image can be improved.

Note that the case where the number of sub-images J is determined to be2 in the procedure 2 and it is determined that T₁=T₂=T/2 in theprocedure 3 has been described here, the present invention is notlimited to this obviously.

For example, in the case where it is determined that T₁<T₂ in theprocedure 3 in the second step, the first sub-image can be brightenedand the second sub-image can be darkened. Further, in the case where itis determined that T₁>T₂ in the procedure 3 in the second step, thefirst sub-image can be darkened and the second sub-image can bebrightened. Thus, the display method can be pseudo impulse driving,while the original image can be perceived by human eyes; therefore,quality of moving images can be improved. Note that as in the case ofthe above-mentioned driving method, when black data insertion isselected among methods in which brightness of the original image isdistributed to a plurality of sub-images in the procedure 1, thesub-image may be directly displayed without changing the brightness ofthe sub-image. This is because when the brightness of the sub-image isnot changed, the original image is merely displayed in such a mannerthat entire brightness of the original image is low. In other words,when this method is positively used for controlling the brightness ofthe display device, brightness can be controlled and the quality ofmoving images increases at the same time.

Further, it is obvious that the number of sub-images J may be anothervalue instead of 2 in the procedure 2. Since advantages in that casehave been already described, detailed description is omitted here. Inthe procedure 1 in the second step, even when black data insertion isselected among methods in which brightness of the original image isdistributed to a plurality of sub-images, it is obvious that similaradvantages can be gained. For example, image quality can be improved byapplying the driving method to the liquid crystal display device inwhich the response time of the liquid crystal element is approximately(1/(J times the conversion ratio)) of a cycle of input image data.

Next, another example of the driving method determined by the procedurein the second step will be described.

In the procedure 1 in the second step, when a time ratio gray scalecontrolling method is selected among methods in which brightness of theoriginal image is distributed to a plurality of sub-images, the drivingmethod is as follows.

One feature of a driving method of the display device is that i-th (i isa positive integer) image data and (i+1)th image data are sequentiallyprepared in a constant cycle T. The cycle T is divided into J (J is aninteger equal to or more than 2) sub-image display periods. The i-thimage data is data which can make each of a plurality of pixels haveunique brightness L. The maximum value of the unique brightness L isL_(max). The j-th (j is an integer equal to or more than 1, and equal toor less than J) sub-image is formed by arranging a plurality of pixelseach having unique brightness L_(j) and is an image which is displayedonly during the j-th sub-image display period T_(j). The aforementionedL, T, L_(j), and T_(j) satisfy the sub-image distribution condition.When the unique brightness L is displayed, the brightness is adjusted inthe range of from (j−1)×L_(max)/J to J×L_(max)/J by adjusting brightnessin only one sub-image display period among the J sub-image displayperiods. Here, as image data which are prepared sequentially in aconstant cycle T, the original image data which is formed in the firststep can be used. That is, all display patterns given in the descriptionof the first step can be combined with the above mentioned drivingmethod.

It is obvious that the driving method can be implemented by combiningvarious values of n and m which are used in the first step.

Then, when the number of sub-images J is determined to be 2 in theprocedure 2 in the second step, and it is determined that T₁=T₂=T/2 inthe procedure 3, the driving method can be as shown in FIG. 71. Sincefeatures and advantages of the driving method (display timing usingvarious values of n and m) shown in FIG. 71 have already been described,detailed description is omitted here. In the procedure 1 in the secondstep, even when the time ratio gray scale controlling method is selectedamong methods in which brightness of the original image is distributedto a plurality of sub-images, it is obvious similar advantages can begained. For example, when an interpolated image in the first step is anintermediate image obtained by motion compensation, motion of a movingimage can be smooth; thus, quality of moving images can be significantlyimproved. The quality of moving images can be improved when the displayframe rate is high, and power consumption and a production cost can bereduced when the display frame rate is low. Further, when a displaydevice is an active matrix liquid crystal display device, a problem ofshortage of writing voltage due to dynamic capacitance can be avoided;thus, quality of moving images can be significantly improved whiledefects, in particular, such as a phenomenon of a moving image in whichtraces are seen and an afterimage are reduced. Flickers which appear byalternating-current driving can be reduced so as not to be perceived byhuman eyes.

In the procedure 1 in the second step, as a typical advantage ofselecting the time ratio gray scale controlling method among methods inwhich brightness of the original image is distributed to a plurality ofsub-images, a circuit operation which produces an intermediate image bymotion compensation can be stopped, or the circuit itself can be omittedfrom the device, whereby power consumption and a production cost of thedevice can be reduced. Further, since the display method can be pseudoimpulse driving, quality of a moving image can be improved, and sincebrightness of the display device does not become lower, powerconsumption can be further reduced.

Note that although the case where the number of sub-images J isdetermined to be 2 in the procedure 2 and it is determined thatT₁=T₂=T/2 in the procedure 3 has been described here, the presentinvention is not limited to this obviously.

For example, in the case where it is determined that T₁<T₂ in theprocedure 3 in the second step, the first sub-image can be brightenedand the second sub-image can be darkened. Further, in the case where itis determined that T₁>T₂ in the procedure 3 in the second step, thefirst sub-image can be darkened and the second sub-image can bebrightened. Thus, the display method can be pseudo impulse driving,while the original image can be perceived by human eyes; therefore,quality of moving image can be improved.

Further, it is obvious that the number of sub-images J may be anothervalue instead of 2 in the procedure 2. Since advantages in that casehave been already described, detailed description is omitted here. Inthe procedure 1 in the second step, even when the time ratio gray scalecontrolling method is selected among methods in which brightness of theoriginal image is distributed to a plurality of sub-images, it isobvious similar advantages can be gained. For example, image quality canbe improved by applying the driving method to the liquid crystal displaydevice in which the response time of the liquid crystal element isapproximately (1/(J times the conversion ratio)) of a cycle of inputimage data.

Next, another example of the driving method determined by the procedurein the second step will be described.

In the procedure 1 in the second step, when gamma complement is selectedamong methods in which brightness of the original image is distributedto a plurality of sub-images, the driving method is as follows.

One feature of a driving method of the display device is that i-th (i isa positive integer) image data and (i+1)th image data are sequentiallyprepared in a constant cycle T. The cycle T is divided into J (J is aninteger equal to or more than 2) sub-image display periods. The i-thimage data is data which can make each of a plurality of pixels haveunique brightness L. The j-th (j is an integer equal to or more than 1,and equal to or less than J) sub-image is formed by arranging aplurality of pixels each having unique brightness L_(j), and is an imagewhich is displayed only during the j-th sub-image display period T_(j).The aforementioned L, T, L_(j), and T_(j) satisfy the sub-imagedistribution condition. In each sub-image, characteristics of a changeof brightness with respect to the gray scale is changed from the linearshape, and total amount of brightness which is changed to a blighterarea from the linear shape and the total amount of brightness which ischanged to a darker area from the linear shape are almost the same inall gray scale. Here, as image data which are prepared sequentially in aconstant cycle T, the original image data which is formed in the firststep can be used. That is, all display patterns given in the descriptionof the first step can be combined with the above-mentioned drivingmethod.

It is obvious that the driving method can be implemented by combiningvarious values of n and m which are used in the first step.

Then, when the number of sub-images J is determined to be 2 in theprocedure 2 in the second step, and it is determined that T₁=T₂=T/2 inthe procedure 3, the driving method can be as shown in FIG. 71. Sincefeatures and advantages of the driving method (display timing usingvarious values of n and m) shown in FIG. 71 have already been described,detailed description is omitted here. In the procedure 1 in the secondstep, even when gamma complement is selected among methods in whichbrightness of the original image is distributed to a plurality ofsub-images, it is obvious similar advantages can be gained. For example,when an interpolated image in the first step is an intermediate imageobtained by motion compensation, motion of moving images can be smooth;thus, quality of moving images can be significantly improved. Thequality of moving images can be improved when the display frame rate ishigh, and power consumption and a production cost can be reduced whenthe display frame rate is low. Further, when a display device is anactive matrix liquid crystal display device, a problem of shortage ofwriting voltage due to dynamic capacitance can be avoided; thus, qualityof moving images can be significantly improved while defects, inparticular, such as a phenomenon of a moving image in which traces areseen and an afterimage are reduced. Flickers which appear byalternating-current driving can be reduced so as not to be perceived byhuman eyes.

In the procedure 1 in the second step, as a typical advantage ofselecting gamma complement among methods in which brightness of theoriginal image is distributed to a plurality of sub-images, a circuitoperation which produces an intermediate image by motion compensationcan be stopped, or the circuit itself can be omitted from the device,whereby power consumption and a production cost of the device can bereduced. Further, since the display method can be pseudo impulse drivingregardless of the gray scale value included in the image data, qualityof a moving image can be improved. Moreover, image data may be directlysubjected to gamma conversion to obtain a sub-image. In this case, thereis an advantage in that the gamma value can be controlled variously bythe amount of movement of a moving image. Further, without the imagedata being directly subjected to gamma conversion, a sub-image whosegamma value is changed may be obtained by change of the referencevoltage of a digital-to-analog converter circuit (DAC). In this case,since the image data is not directly subjected to gamma conversion, acircuit operation for gamma conversion can be stopped, or the circuititself can be omitted from the device, whereby power consumption and aproduction cost of the device can be reduced. Further, in gammacomplement, since the change of the brightness L_(j) of each sub-imagewith respect to gray scale follows a gamma curve, the gray scale of eachsub-image can be displayed smoothly by itself; therefore, there is anadvantage in that image quality to be perceived in the end by human eyesis improved.

Note that although the case where the number of sub-images J isdetermined to be 2 in the procedure 2 and it is determined thatT₁=T₂=T/2 in the procedure 3 has been described here, the presentinvention is not limited to this obviously.

For example, in the case where it is determined that T₁<T₂ in theprocedure 3 in the second step, the first sub-image can be brightenedand the second sub-image can be darkened. Further, in the case where itis determined that T₁>T₂ in the procedure 3 in the second step, thefirst sub-image can be darkened and the second sub-image can bebrightened. Thus, the display method can be pseudo impulse driving,while the original image can be perceived by human eyes; therefore,quality of moving images can be improved. In the procedure 1, when gammacomplement is selected among methods in which brightness of the originalimage is distributed to a plurality of sub-images as in the case of theabove-mentioned driving method, the gamma value may be changed in thecase where brightness of the sub-image is changed. That is, the gammavalue may be determined in accordance with display timing of the secondsub-image. Accordingly, the operation of a circuit for changingbrightness of the entire image can be stopped, or the circuit itself canbe omitted from the device, whereby power consumption and a productioncost of the device can be reduced.

Further, it is obvious that the number of sub-images J may be anothervalue instead of 2 in the procedure 2. Since advantages in that casehave been already described, detailed description is omitted here. Inthe procedure 1 in the second step, even when gamma complement isselected among methods in which brightness of the original image isdistributed to a plurality of sub-images, it is obvious similaradvantages can be gained. For example, image quality can be improved byapplying the driving method to the liquid crystal display device inwhich the response time of the liquid crystal element is approximately(1/(J times the conversion ratio)) of a cycle of input image data.

Next, another example of the driving method determined by the procedurein the second step will be described in detail.

When a method in which an intermediate image obtained by motioncompensation is used as a sub-image is selected in the procedure 1 inthe second step; when the number of sub-images is determined to be 2 inthe procedure 2 in the second step; and when it is determined thatT₁=T₂=T/2 in the procedure 3 in the second step, the driving methoddetermined by the procedures in the second step can be as follows.

One feature of a driving method of the display device is that i-th (i isa positive integer) image data and (i+1)th image data are sequentiallyprepared in a constant cycle T. A k-th (k is a positive integer) image,a (k+1)th image, and a (k+2)th image are sequentially displayed at halfinterval of the period of the original image data. The k-th image isdisplayed in accordance with the i-th image data. The (k+1)th image isdisplayed in accordance with the image data which corresponds to halfamount of the movement of from the i-th image data to the (i+1)th imagedata. The (k+2)th image is displayed in accordance with the (i+1)thimage data. Here, as the image data which are prepared sequentially in aconstant cycle T, the original image data which is formed in the firststep can be used. That is, all display patterns given in the descriptionof the first step can be combined with the above-mentioned drivingmethod.

It is obvious that the driving method can be implemented by combiningvarious values of n and m which are used in the first step.

In the procedure 1 in the second step, a typical advantage of selectinga method in which an intermediate image obtained by motion compensationis used as a sub-image is that a method for obtaining an intermediateimage employed in the first step can be similarly used in the secondstep when an intermediate image obtained by motion compensation is aninterpolated image. In other words, a circuit for obtaining anintermediate image by motion compensation can be used not only in thefirst step, but also in the second step, whereby the circuit can be usedefficiently and treatment efficiency can be increased. In addition,motion of moving images can be further smooth; thus, quality of movingimages can be further improved.

Note that although the case where the number of sub-images J isdetermined to be 2 in the procedure 2 and it is determined thatT₁=T₂=T/2 in the procedure 3 has been described here, the presentinvention is not limited to this obviously.

For example, in the case where it is determined that T₁<T₂ in theprocedure 3 in the second step, the first sub-image can be brightenedand the second sub-image can be darkened. Further, in the case where itis determined that T₁>T₂ in the procedure 3 in the second step, thefirst sub-image can be darkened and the second sub-image can bebrightened. Thus, the display method can be pseudo impulse driving,while the original image can be perceived by human eyes; therefore,quality of moving images can be improved. Note that as in the case ofthe above-mentioned driving method, when a method in which anintermediate image obtained by motion compensation is used as asub-image is selected in the procedure 2, it is not necessary thatbrightness of the sub-image is changed. This is because the image in anintermediate state is completed as an image in itself, and even whendisplay timing of the second sub-image is changed, the image which isperceived by human eyes is not changed. In this case, the operation of acircuit for changing brightness of the entire image can be stopped, orthe circuit itself can be omitted from the device, whereby powerconsumption and a production cost of the device can be reduced.

Further, it is obvious that the number of sub-images J may be anothervalue instead of 2 in the procedure 2. Since advantages in that casehave been already described, detailed description is omitted here. Inthe procedure 1 in the second step, even when a method in which anintermediate image obtained by motion compensation is used as asub-image is selected, it is obvious similar advantages can be gained.For example, image quality can be improved by applying the drivingmethod to the liquid crystal display device in which the response timeof the liquid crystal element is approximately (1/(J times theconversion ratio)) of a cycle of input image data.

Next, specific examples of a method for converting the frame rate whenthe input frame rate and the display frame rate are different aredescribed with reference to FIGS. 73A to 73C. In methods shown in FIGS.73A to 73C, circular regions in images are changed from frame to frame,and triangle regions in the images are hardly changed from frame toframe. Note that the images are just examples for explanation, and theimages to be displayed are not limited to these examples. The methodsshown in FIGS. 73A to 73C can be applied to various images.

FIG. 73A shows the case where the display frame rate is twice as high asthe input frame rate (the conversion ratio is 2). When the conversionratio is 2, there is an advantage in that quality of moving images canbe improved compared to the case where the conversion ratio is less than2. Further, when the conversion ratio is 2, there is an advantage inthat power consumption and manufacturing cost can be reduced compared tothe case where the conversion ratio is more than 2. FIG. 73Aschematically shows time change in images to be displayed with timerepresented by the horizontal axis. Here, a focused image is referred toas a p-th image (p is a positive integer). An image displayed after thefocused image is referred to as a (p+1)th image, and an image displayedbefore the focused image is referred to as a (p−1)th image, for example.Thus, how far an image to be displayed is apart from the focused imageis described for convenience. An image 7301 is the p-th image; an image7302 is the (p+1)th image; an image 7303 is a (p+2)th image; an image7304 is a (p+3)th image; and an image 7305 is a (p+4)th image. Theperiod T_(in) shows a cycle of input image data. Note that since FIG.73A shows the case where the conversion ratio is 2, the period T_(in) istwice as long as a period after the p-th image is displayed until the(p+1)th image is displayed.

Here, the (p+1)th image 7302 may be an image which is made to be in anintermediate state between the p-th image 7301 and the (p+2)th image7303 by detecting the amount of change in the images from the p-th image7301 to the (p+2)th image 7303. FIG. 73A shows an image in anintermediate state by a region whose position is changed from frame toframe (the circular region) and a region whose position is hardlychanged from frame to frame (the triangle region). In other words, theposition of the circular region in the (p+1)th image 7302 is anintermediate position between the positions of the circular regions inthe p-th image 7301 and the (p+2)th image 7303. That is, as for the(p+1)th image 7302, image data is interpolated by motion compensation.When motion compensation is performed on a moving object on the image inthis manner to interpolate the image data, smooth display can beperformed.

Further, the (p+1)th image 7302 may be an image which is made to be inan intermediate state between the p-th image 7301 and the (p+2)th image7303 and may be an image, luminance of which is controlled by a certainrule. As the certain rule, for example, L>L_(c) may be satisfied whentypical luminance of the p-th image 7301 is denoted by L and typicalluminance of the (p+1)th image 7302 is denoted by L_(c), as shown inFIG. 73A. Preferably, 0.1L<L_(c)<0.8L is satisfied, and more preferably0.2L<L_(c)<0.5L is satisfied. Alternatively, L<L_(c) may be satisfied,preferably 0.1L_(c)<L<0.8L_(c) is satisfied, and more preferably0.2L_(c)<L<0.5L_(c) is satisfied. In this manner, display can be madepseudo impulse display, so that an afterimage perceived by human eyescan be suppressed.

Note that typical luminance of the images is described later in detailwith reference to FIGS. 74A to 74E.

When two different causes of motion blur (non-smoothness in movement ofimages and an afterimage perceived by human eyes) are removed at thesame time in this manner, motion blur can be considerably reduced.

Moreover, the (p+3)th image 7304 may also be formed from the (p+2)thimage 7303 and the (p+4)th image 7305 by using a similar method. Thatis, the (p+3)th image 7304 may be an image which is made to be in anintermediate state between the (p+2)th image 7303 and the (p+4)th image7305 by detecting the amount of change in the images from the (p+2)thimage 7303 to the (p+4)th image 7305 and may be an image, luminance ofwhich is controlled by a certain rule.

FIG. 73B shows the case where the display frame rate is three times ashigh as the input frame rate (the conversion ratio is 3). FIG. 73Bschematically shows time change in images to be displayed with timerepresented by the horizontal axis. An image 7311 is the p-th image; animage 7312 is the (p+1)th image; an image 7313 is a (p+2)th image; animage 7314 is a (p+3)th image; an image 7315 is a (p+4)th image; animage 7316 is a (p+5)th image; and an image 7317 is a (p+6)th image. Theperiod T_(in) shows a cycle of input image data. Note that since FIG.73B shows the case where the conversion ratio is 3, the period T_(in) isthree times as long as a period after the p-th image is displayed untilthe (p+1)th image is displayed.

Here, each of the (p+1)th image 7312 and the (p+2)th image 7313 may bean image which is made to be in an intermediate state between the p-thimage 7311 and the (p+3)th image 7314 by detecting the amount of changein the images from the p-th image 7311 to the (p+3)th image 7314. FIG.73B shows an image in an intermediate state by a region whose positionis changed from frame to frame (the circular region) and a region whoseposition is hardly changed from frame to frame (the triangle region).That is, the position of the circular region in each of the (p+1)thimage 7312 and the (p+2)th image 7313 is an intermediate positionbetween the positions of the circular regions in the p-th image 7311 andthe (p+3)th image 7314. Specifically, when the amount of movement of thecircular regions detected from the p-th image 7311 and the (p+3)th image7314 is denoted by X, the position of the circular region in the (p+1)thimage 7312 may be displaced by approximately (1/3)X from the position ofthe circular region in the p-th image 7311. Further, the position of thecircular region in the (p+2)th image 7313 may be displaced byapproximately (2/3)X from the position of the circular region in thep-th image 7311. That is, as for each of the (p+1)th image 7312 and the(p+2)th image 7313, image data is interpolated by motion compensation.When motion compensation is performed on a moving object on the image inthis manner to interpolate the image data, smooth display can beperformed.

Further, each of the (p+1)th image 7312 and the (p+2)th image 7313 maybe an image which is made to be in an intermediate state between thep-th image 7311 and the (p+3)th image 7314 and may be an image,luminance of which is controlled by a certain rule. As the certain rule,for example, L>L_(c)1, L>L_(c)2, or L_(c)1=L_(c)2 may be satisfied whentypical luminance of the p-th image 7311 is denoted by L, typicalluminance of the (p+1)th image 7312 is denoted by L_(c)1, and typicalluminance of the (p+2)th image 7313 is denoted by L_(c)2, as shown inFIG. 73B. Preferably, 0.1L<L_(c)1=L_(c)2<0.8L, is satisfied, and morepreferably 0.2L<L_(c)1=L_(c)2<0.5L is satisfied. Alternatively,L<L_(c)1, L<L_(c)2, or L_(c)1=L_(c)2 may be satisfied, preferably0.1L_(c)1=0.1L_(c)2<L<0.8L_(c)1=0.8L_(c)2 is satisfied, and morepreferably 0.2L_(c)1=0.2L_(c)2<L<0.5L_(c)1=0.5L_(c)2 is satisfied. Inthis manner, display can be made pseudo impulse display, so that anafterimage perceived by human eyes can be suppressed. Alternatively,images, luminance of which is changed, may be made to appearalternately. In this manner, a cycle of luminance change can beshortened, so that flickers can be reduced.

When two different causes of motion blur (non-smoothness in movement ofimages and an afterimage perceived by human eyes) are removed at thesame time in this manner, motion blur can be considerably reduced.

Moreover, each of the (p+4)th image 7315 and the (p+5)th image 7316 mayalso be formed from the (p+3)th image 7314 and the (p+6)th image 7317 byusing a similar method. That is, each of the (p+4)th image 7315 and the(p+5)th image 7316 may be an image which is made to be in anintermediate state between the (p+3)th image 7314 and the (p+6)th image7317 by detecting the amount of change in the images from the (p+3)thimage 7314 to the (p+6)th image 7317 and may be an image, luminance ofwhich is controlled by a certain rule.

Note that when the method shown in FIG. 73B is used, the display framerate is so high that movement of the image can follow movement of humaneyes, so that movement of the image can be displayed smoothly.Therefore, motion blur can be considerably reduced.

FIG. 73C shows the case where the display frame rate is 1.5 times ashigh as the input frame rate (the conversion ratio is 1.5). FIG. 73Cschematically shows time change in images to be displayed with timerepresented by the horizontal axis. An image 7321 is the p-th image; animage 7322 is the (p+1)th image; an image 7323 is the (p+2)th image; andan image 7324 is the (p+3)th image. Note that although not necessarilydisplayed actually, an image 7325, which is input image data, may beused to form the (p+1)th image 7322 and the (p+2)th image 7323. Theperiod T_(in) shows a cycle of input image data. Note that since FIG.73C shows the case where the conversion ratio is 1.5, the period T_(in)is 1.5 times as long as a period after the p-th image is displayed untilthe (p+1)th image is displayed.

Here, each of the (p+1)th image 7322 and the (p+2)th image 7323 may bean image which is made to be in an intermediate state between the p-thimage 7321 and the (p+3)th image 7324 by detecting the amount of changein the images from the p-th image 7321 to the (p+3)th image 7324 via theimage 7325. FIG. 73C shows an image in an intermediate state by a regionwhose position is changed from frame to frame (the circular region) anda region whose position is hardly changed from frame to frame (thetriangle region). That is, the position of the circular region in eachof the (p+1)th image 7322 and the (p+2)th image 7323 is an intermediateposition between the positions of the circular regions in the p-th image7321 and the (p+3)th image 7324. That is, as for each of the (p+1)thimage 7322 and the (p+2)th image 7323, image data is interpolated bymotion compensation. When motion compensation is performed on a movingobject on the image in this manner to interpolate the image data, smoothdisplay can be performed.

Further, each of the (p+1)th image 7322 and the (p+2)th image 7323 maybe an image which is made to be in an intermediate state between thep-th image 7321 and the (p+3)th image 7324 and may be an image,luminance of which is controlled by a certain rule. As the certain rule,for example, L>L_(c)1, L>L_(c)2, or L_(c)1=L_(c)2 is satisfied whentypical luminance of the p-th image 7321 is denoted by L, typicalluminance of the (p+1)th image 7322 is denoted by L_(c)1, and typicalluminance of the (p+2)th image 7323 is denoted by L_(c)2, as shown inFIG. 73C. Preferably, 0.1L<L_(c)1=L_(c)2<0.8L is satisfied, and morepreferably 0.2L<L_(c)1=L_(c)2<0.5L is satisfied. Alternatively,L<L_(c)1, L<L_(c)2, or L_(c)1=L_(c)2 may be satisfied, preferably0.1L_(c)1=0.1L_(c)2<L<0.8L_(c)1=0.8L_(c)2 is satisfied, and morepreferably 0.2L_(c)1=0.2L_(c)2<L<0.5L_(c)1=0.5L_(c)2 is satisfied. Inthis manner, display can be made pseudo impulse display, so that anafterimage perceived by human eyes can be suppressed. Alternatively,images, luminance of which is changed, may be made to appearalternately. In this manner, a cycle of luminance change can beshortened, so that flickers can be reduced.

When two different causes of motion blur (non-smoothness in movement ofimages and an afterimage perceived by human eyes) are removed at thesame time in this manner, motion blur can be considerably reduced.

Note that when the method shown in FIG. 73C is used, the display framerate is so low that time for writing a signal to a display device can beincreased. Therefore, clock frequency of the display device can be madelower, so that power consumption can be reduced. Further, processingspeed of motion compensation can be decreased, so that power consumptioncan be reduced.

Next, typical luminance of images is described with reference to FIGS.74A to 74E. FIGS. 74A to 74D each schematically show time change inimages to be displayed with time represented by the horizontal axis FIG.74E shows an example of a method for measuring luminance of an image ina certain region.

An example of a method for measuring luminance of an image is a methodfor individually measuring luminance of each pixel which forms theimage. With this method, luminance in every detail of the image can bestrictly measured.

Note that since a method for individually measuring luminance of eachpixel which forms the image needs much energy, another method may beused. An example of another method for measuring luminance of an imageis a method for measuring average luminance of a region in an image,which is focused. With this method, luminance of an image can be easilymeasured. In this embodiment mode, luminance measured by a method formeasuring average luminance of a region in an image is referred to astypical luminance of an image for convenience.

Then, which region in an image is focused in order to measure typicalluminance of the image is described below.

FIG. 74A shows an example of a measuring method in which luminance of aregion whose position is hardly changed with respect to change in animage (the triangle region) is typical luminance of the image. Theperiod T_(in) shows a cycle of input image data; an image 7401 is thep-th image; an image 7402 is the (p+1)th image; an image 7403 is the(p+2)th image; a first region 7404 is a luminance measurement region inthe p-th image 7401; a second region 7405 is a luminance measurementregion in the (p+1)th image 7402; and a third region 7406 is a luminancemeasurement region in the (p+2)th image 7403. Here, the first to thirdregions may be provided in almost the same spatial positions in adevice. That is, when typical luminance of the images is measured in thefirst to third regions, time change in typical luminance of the imagescan be calculated.

When the typical luminance of the images is measured, whether display ismade pseudo impulse display or not can be judged. For example, ifL_(c)<L is satisfied when luminance measured in the first region 7404 isdenoted by L and luminance measured in the second region 7405 is denotedby L_(c), it can be said that display is made pseudo impulse display. Atthat time, it can be said that quality of a moving image is improved.

Note that when the amount of change in typical luminance of the imageswith respect to time change (relative luminance) in the luminancemeasurement regions is in the following range, image quality can beimproved. As for relative luminance, for example, relative luminancebetween the first region 7404 and the second region 7405 can be theratio of lower luminance to higher luminance; relative luminance betweenthe second region 7405 and the third region 7406 can be the ratio oflower luminance to higher luminance; and relative luminance between thefirst region 7404 and the third region 7406 can be the ratio of lowerluminance to higher luminance. That is, when the amount of change intypical luminance of the images with respect to time change (relativeluminance) is 0, relative luminance is 100%. When the relative luminanceis less than or equal to 80%, quality of a moving image can be improved.In particular, when the relative luminance is less than or equal to 50%,quality of a moving image can be significantly improved. Further, whenthe relative luminance is more than or equal to 10%, power consumptionand flickers can be reduced. In particular, when the relative luminanceis more than or equal to 20%, power consumption and flickers can besignificantly reduced. That is, when the relative luminance is more thanor equal to 10% and less than or equal to 80%, quality of a moving imagecan be improved and power consumption and flickers can be reduced.Further, when the relative luminance is more than or equal to 20% andless than or equal to 50%, quality of a moving image can besignificantly improved and power consumption and flickers can besignificantly reduced.

FIG. 74B shows an example of a method in which luminance of regionswhich are divided into tiled shapes is measured and an average valuethereof is typical luminance of an image. The period T_(in) shows acycle of input image data; an image 7411 is the p-th image; an image7412 is the (p+1)th image; an image 7413 is the (p+2)th image; a firstregion 7414 is a luminance measurement region in the p-th image 7411; asecond region 7415 is a luminance measurement region in the (p+1)thimage 7412; and a third region 7416 is a luminance measurement region inthe (p+2)th image 7413. Here, the first to third regions may be providedin almost the same spatial positions in a device. That is, when typicalluminance of the images is measured in the first to third regions, timechange in typical luminance of the images can be measured.

When the typical luminance of the images is measured, whether display ismade pseudo impulse display or not can be judged. For example, ifL_(c)<L is satisfied when luminance measured in the first region 7414 isdenoted by L and luminance measured in the second region 7415 is denotedby Lc, it can be said that display is made pseudo impulse display. Atthat time, it can be said that quality of a moving image is improved.

Note that when the amount of change in typical luminance of the imageswith respect to time change (relative luminance) in the luminancemeasurement regions is in the following range, image quality can beimproved. As for relative luminance, for example, relative luminancebetween the first region 7414 and the second region 7415 can be theratio of lower luminance to higher luminance; relative luminance betweenthe second region 7415 and the third region 7416 can be the ratio oflower luminance to higher luminance; and relative luminance between thefirst region 7414 and the third region 7416 can be the ratio of lowerluminance to higher luminance. That is, when the amount of change intypical luminance of the images with respect to time change (relativeluminance) is 0, relative luminance is 100%. When the relative luminanceis less than or equal to 80%, quality of a moving image can be improved.In particular, when the relative luminance is less than or equal to 50%,quality of a moving image can be significantly improved. Further, whenthe relative luminance is more than or equal to 10%, power consumptionand flickers can be reduced. In particular, when the relative luminanceis more than or equal to 20%, power consumption and flickers can besignificantly reduced. That is, when the relative luminance is more thanor equal to 10% and less than or equal to 80%, quality of a moving imagecan be improved and power consumption and flickers can be reduced.Further, when the relative luminance is more than or equal to 20% andless than or equal to 50%, quality of a moving image can besignificantly improved and power consumption and flickers can besignificantly reduced.

FIG. 74C shows an example of a method in which luminance of a centerregion in an image is measured and an average value thereof is typicalluminance of the image. The period T_(in) shows a cycle of input imagedata; an image 7421 is the p-th image; an image 7422 is the (p+1)thimage; an image 7423 is the (p+2)th image; a first region 7424 is aluminance measurement region in the p-th image 7421; a second region7425 is a luminance measurement region in the (p+1)th image 7422; and athird region 7426 is a luminance measurement region in the (p+2)th image7423.

When the typical luminance of the images is measured, whether display ismade pseudo impulse display or not can be judged. For example, ifL_(c)<L is satisfied when luminance measured in the first region 7424 isdenoted by L and luminance measured in the second region 7425 is denotedby L_(c), it can be said that display is made pseudo impulse display. Atthat time, it can be said that quality of a moving image is improved.

Note that when the amount of change in typical luminance of the imageswith respect to time change (relative luminance) in the luminancemeasurement regions is in the following range, image quality can beimproved. As for relative luminance, for example, relative luminancebetween the first region 7424 and the second region 7425 can be theratio of lower luminance to higher luminance; relative luminance betweenthe second region 7425 and the third region 7426 can be the ratio oflower luminance to higher luminance; and relative luminance between thefirst region 7424 and the third region 7426 can be the ratio of lowerluminance to higher luminance. That is, when the amount of change intypical luminance of the images with respect to time change (relativeluminance) is 0, relative luminance is 100%. When the relative luminanceis less than or equal to 80%, quality of a moving image can be improved.In particular, when the relative luminance is less than or equal to 50%,quality of a moving image can be significantly improved. Further, whenthe relative luminance is more than or equal to 10%, power consumptionand flickers can be reduced. In particular, when the relative luminanceis more than or equal to 20%, power consumption and flickers can besignificantly reduced. That is, when the relative luminance is more thanor equal to 10% and less than or equal to 80%, quality of a moving imagecan be improved and power consumption and flickers can be reduced.Further, when the relative luminance is more than or equal to 20% andless than or equal to 50%, quality of a moving image can besignificantly improved and power consumption and flickers can besignificantly reduced.

FIG. 74D shows an example of a method in which luminance of a pluralityof points sampled from the entire image is measured and an average valuethereof is typical luminance of the image. The period T_(in) shows acycle of input image data; an image 7431 is the p-th image; an image7432 is the (p+1)th image; an image 7433 is the (p+2)th image; a firstregion 7434 is a luminance measurement region in the p-th image 7431; asecond region 7435 is a luminance measurement region in the (p+1)thimage 7432; and a third region 7436 is a luminance measurement region inthe (p+2)th image 7433.

When the typical luminance of the images is measured, whether display ismade pseudo impulse display or not can be judged. For example, ifL_(c)<L is satisfied when luminance measured in the first region 7434 isdenoted by L and luminance measured in the second region 7435 is denotedby L_(c), it can be said that display is made pseudo impulse display. Atthat time, it can be said that quality of a moving image is improved.

Note that when the amount of change in typical luminance of the imageswith respect to time change (relative luminance) in the luminancemeasurement regions is in the following range, image quality can beimproved. As for relative luminance, for example, relative luminancebetween the first region 7434 and the second region 7435 can be theratio of lower luminance to higher luminance; relative luminance betweenthe second region 7435 and the third region 7436 can be the ratio oflower luminance to higher luminance; and relative luminance between thefirst region 7434 and the third region 7436 can be the ratio of lowerluminance to higher luminance That is, when the amount of change intypical luminance of the images with respect to time change (relativeluminance) is 0, relative luminance is 100%. When the relative luminanceis less than or equal to 80%, quality of a moving image can be improved.In particular, when the relative luminance is less than or equal to 50%,quality of a moving image can be significantly improved. Further, whenthe relative luminance is more than or equal to 10%, power consumptionand flickers can be reduced. In particular, when the relative luminanceis more than or equal to 20%, power consumption and flickers can besignificantly reduced. That is, when the relative luminance is more thanor equal to 10% and less than or equal to 80%, quality of a moving imagecan be improved and power consumption and flickers can be reduced.Further, when the relative luminance is more than or equal to 20% andless than or equal to 50%, quality of a moving image can besignificantly improved and power consumption and flickers can besignificantly reduced.

FIG. 74E shows a measurement method in the luminance measurement regionsshown in FIGS. 74A to 74D. A region 7441 is a focused luminancemeasurement region, and a point 7442 is a luminance measurement point inthe region 7441. In a luminance measurement apparatus having high timeresolution, a measurement range thereof is small in some cases.Therefore, in the case where the region 7441 is large, unlike the caseof measuring the whole region, a plurality of points in the region 7441may be measured uniformly by dots and an average value thereof may bethe luminance of the region 744, as shown in FIG. 74E.

Note that in the case where the image is formed using combination ofthree primary colors of R, G, and B, luminance to be measured may beluminance of R, and B, luminance of R and C₃ luminance of G and B,luminance of B and R, or each luminance of R, G, and B.

Next, a method for producing an image in an intermediate state bydetecting movement of an image, which is included in input image data,and a method for controlling a driving method in accordance withmovement of an image, which is included in input image data, or the likeare described.

A method for producing an image in an intermediate state by detectingmovement of an image, which is included in input image data, isdescribed with reference to FIGS. 75A and 75B. FIG. 75A shows the casewhere the display frame rate is twice as high as the input frame rate(the conversion ratio is 2). FIG. 75A schematically shows a method fordetecting movement of an image with time represented by the horizontalaxis. The period T_(in) shows a cycle of input image data; an image 7501is the p-th image; an image 7502 is the (p+1)th image; and an image 7503is the (p+2)th image. Further, as regions which are independent of time,a first region 7504, a second region 7505, and a third region 7506 areprovided in images.

First, in the (p+2)th image 7503, the image is divided into a pluralityof tiled regions, and image data in the third region 7506 which is oneof the regions is focused.

Next, in the p-th image 7501, a region which uses the third region 7506as the center and is larger than the third region 7506 is focused. Here,the region which uses the third region 7506 as the center and is largerthan the third region 7506 corresponds to a data retrieval region. Inthe data retrieval region, a range in a horizontal direction (an Xdirection) is denoted by 7507 and a range in a perpendicular direction(a Y direction) is denoted by 7508. Note that the range in thehorizontal direction 7507 and the range in the perpendicular direction7508 may be ranges in which each of a range in a horizontal directionand a range in a perpendicular direction of the third region 7506 isenlarged by approximately 15 pixels.

Then, in the data retrieval region, a region having image data which ismost similar to the image data in the third region 7506 is retrieved. Asa retrieval method, a least-squares method or the like can be used. As aresult of retrieval, it is assumed that the first region 7504 be derivedas the region having the most similar image data.

Next, as an amount which shows positional difference between the derivedfirst region 7504 and the third region 7506, a vector 7509 is derived.Note that the vector 7509 is referred to as a motion vector.

Then, in the (p+1)th image 7502, the second region 7505 is formed by avector calculated from the motion vector 7509, the image data in thethird region 7506 in the (p+2)th image 7503, and image data in the firstregion 7504 in the p-th image 7501.

Here, the vector calculated from the motion vector 7509 is referred toas a displacement vector 7510. The displacement vector 7510 has afunction of determining a position in which the second region 7505 isformed. The second region 7505 is formed in a position which is apartfrom the third region 7506 by the displacement vector 7510. Note thatthe amount of the displacement vector 7510 may be an amount which isobtained by multiplying the motion vector 7509 by a coefficient (1/2).

Image data in the second region 7505 in the (p+1)th image 7502 may bedetermined by the image data in the third region 7506 in the (p+2)thimage 7503 and the image data in the first region 7504 in the p-th image7501. For example, the image data in the second region 7505 in the(p+1)th image 7502 may be an average value between the image data in thethird region 7506 in the (p+2)th image 7503 and the image data in thefirst region 7504 in the p-th image 7501.

In this manner, the second region 7505 in the (p+1)th image 7502, whichcorresponds to the third region 7506 in the (p+2)th image 7503, can beformed. Note that when the above-described treatment is also performedon other regions in the (p+2)th image 7503, the (p+1)th image 7502 whichis made to be in an intermediate state between the (p+2)th image 7503and the p-th image 7501 can be formed.

FIG. 75B shows the case where the display frame rate is three times ashigh as the input frame rate (the conversion ratio is 3). FIG. 75Bschematically shows a method for detecting movement of an image withtime represented by the horizontal axis. The period T_(in) shows a cycleof input image data; an image 7511 is the p-th image; an image 7512 isthe (p+1)th image; an image 7513 is the (p+2)th image; and an image 7514is the (p+3)th image. Further, as regions which are independent of time,a first region 7515, a second region 7516, a third region 7517, and afourth region 7518 are provided in images.

First, in the (p+3)th image 7514, the image is divided into a pluralityof tiled regions, and image data in the fourth region 7518 which is oneof the regions is focused.

Next, in the p-th image 7511, a region which uses the fourth region 7518as the center and is larger than the fourth region 7518 is focused.Here, the region which uses the fourth region 7518 as the center and islarger than the fourth region 7518 corresponds to a data retrievalregion. In the data retrieval region, a range in a horizontal direction(an X direction) is denoted by 7519 and a range in a perpendiculardirection (a Y direction) is denoted by 7520. Note that the region inthe horizontal direction 7519 and the range in the perpendiculardirection 7520 may be ranges in which each of a range in a horizontaldirection and a range in a perpendicular direction of the fourth region7518 is enlarged by approximately 15 pixels.

Then, in the data retrieval region, a region having image data which ismost similar to the image data in the fourth region 7518 is retrieved.As a retrieval method, a least-squares method or the like can be used.As a result of retrieval, it is assumed that the first region 7515 bederived as the region having the most similar image data.

Next, as an amount which shows positional difference between the derivedfirst region 7515 and the fourth region 7518, a vector 7521 is derived.Note that the vector 7521 is referred to as a motion vector.

Then, in each of the (p+1)th image 7512 and the (p+2)th image 7513, thesecond region 7516 and the third region 7517 are formed by a vector 7522and a vector 7523 calculated from the motion vector 7521, the image datain the fourth region 7518 in the (p+3)th image 7514, and image data inthe first region 7515 in the p-th image 7511.

Here, the vector 7522 calculated from the motion vector 7521 is referredto as a first displacement vector 7522. In addition, the vector 7523 isreferred to as a second displacement vector. The first displacementvector 7522 has a function of determining a position in which the secondregion 7516 is formed. The second region 7516 is formed in a positionwhich is apart from the fourth region 7518 by the first displacementvector 7522. Note that the first displacement vector 7522 may be anamount which is obtained by multiplying the motion vector 7521 by acoefficient (1/3). Further, the second displacement vector 7523 has afunction of determining a position in which the third region 7517 isformed. The third region 7517 is formed in a position which is apartfrom the fourth region 7518 by the second displacement vector 7523. Notethat the second displacement vector 7523 may be an amount which isobtained by multiplying the motion vector 7521 by a coefficient (2/3).

Image data in the second region 7516 in the (p+1)th image 7512 may bedetermined by the image data in the fourth region 7518 in the (p+3)thimage 7514 and the image data in the first region 7515 in the p-th image7511. For example, the image data in the second region 7516 in the(p+1)th image 7512 may be an average value between the image data in thefourth region 7518 in the (p+3)th image 7514 and the image data in thefirst region 7515 in the p-th image 7511.

Image data in the third region 7517 in the (p+2)th image 7513 may bedetermined by the image data in the fourth region 7518 in the (p+3)thimage 7514 and the image data in the first region 7515 in the p-th image7511. For example, the image data in the third region 7517 in the(p+2)th image 7513 may be an average value between the image data in thefourth region 7518 in the (p+3)th image 7514 and the image data in thefirst region 7515 in the p-th image 7511.

In this manner, the second region 7516 in the (p+1)th image 7512 and thethird region 7517 in the (p+2)th image 7513 which correspond to thefourth region 7518 in the (p+3)th image 7514 can be formed. Note thatwhen the above-described treatment is also performed on other regions inthe (p+3)th image 7514, the (p+1)th image 7512 and the (p+2)th image7513 which are made to be in an intermediate state between the (p+3)thimage 7514 and the p-th image 7511 can be formed.

Next, an example of a circuit which produces an image in an intermediatestate by detecting movement of an image, which is included in inputimage data, is described with reference to FIGS. 76A to 76D. FIG. 76Ashows a connection relation between a peripheral driver circuitincluding a source driver and a gate driver for displaying an image on adisplay region, and a control circuit for controlling the peripheraldriver circuit. FIG. 76B shows an example of a specific circuitstructure of the control circuit. FIG. 76C shows an example of aspecific circuit structure of an image processing circuit included inthe control circuit. FIG. 76D shows another example of the specificcircuit structure of the image processing circuit included in thecontrol circuit.

As shown in FIG. 76A, a device in this embodiment mode may include acontrol circuit 7611, a source driver 7612, a gate driver 7613, and adisplay region 7614.

Note that the control circuit 7611, the source driver 7612, and the gatedriver 7613 may be formed over the same substrate as the display region7614.

Note that part of the control circuit 7611, the source driver 7612, andthe gate driver 7613 may be formed over the same substrate as thedisplay region 7614, and other circuits may be formed over a differentsubstrate from that of the display region 7614. For example, the sourcedriver 7612 and the gate driver 7613 may be formed over the samesubstrate as the display region 7614, and the control circuit 7611 maybe formed over a different substrate as an external IC. Similarly, thegate driver 7613 may be formed over the same substrate as the displayregion 7614, and other circuits may be formed over a different substrateas an external IC. Similarly, part of the source driver 7612, the gatedriver 7613, and the control circuit 7611 may be formed over the samesubstrate as the display region 7614, and other circuits may be formedover a different substrate as an external IC.

The control circuit 7611 may have a structure to which an external imagesignal 7600, a horizontal synchronization signal 7601, and a verticalsynchronization signal 7602 are input and an image signal 7603, a sourcestart pulse 7604, a source clock 7605, a gate start pulse 7606, and agate clock 7607 are output.

The source driver 7612 may have a structure in which the image signal7603, the source start pulse 7604, and the source clock 7605 are inputand voltage or current in accordance with the image signal 7603 isoutput to the display region 7614.

The gate driver 7613 may have a structure to which the gate start pulse7606 and the gate clock 7607 are input and a signal which specifiestiming for writing a signal output from the source driver 7612 to thedisplay region 7614 is output.

In the case where frequency of the external image signal 7600 isdifferent from frequency of the image signal 7603, a signal forcontrolling timing for driving the source driver 7612 and the gatedriver 7613 is also different from frequency of the horizontalsynchronization signal 7601 and the vertical synchronization signal 7602which are input. Therefore, in addition to processing of the imagesignal 7603, it is necessary to process the signal for controllingtiming for driving the source driver 7612 and the gate driver 7613. Thecontrol circuit 7611 may have a function of processing the signal forcontrolling timing for driving the source driver 7612 and the gatedriver 7613. For example, in the case where the frequency of the imagesignal 7603 is twice as high as the frequency of the external imagesignal 7600, the control circuit 7611 generates the image signal 7603having twice frequency by interpolating an image signal included in theexternal image signal 7600 and controls the signal for controllingtiming so that the signal also has twice frequency.

Further, as shown in FIG. 76B, the control circuit 7611 may include animage processing circuit 7615 and a timing generation circuit 7616.

The image processing circuit 7615 may have a structure to which theexternal image signal 7600 and a frequency control signal 7608 are inputand the image signal 7603 is output.

The timing generation circuit 7616 may have a structure to which thehorizontal synchronization signal 7601 and the vertical synchronizationsignal 7602 are input, and the source start pulse 7604, the source clock7605, the gate start pulse 7606, the gate clock 7607, and the frequencycontrol signal 7608 are output. Note that the timing generation circuit7616 may have a memory, a register, or the like for holding data forspecifying the state of the frequency control signal 7608.Alternatively, the timing generation circuit 7616 may have a structureto which a signal for specifying the state of the frequency controlsignal 7608 is input from outside.

As shown in FIG. 76C, the image processing circuit 7615 may include amotion detection circuit 7620, a first memory 7621, a second memory7622, a third memory 7623, a luminance control circuit 7624, and ahigh-speed processing circuit 7625.

The motion detection circuit 7620 may have a structure in which aplurality of pieces of image data are input, movement of an image isdetected, and image data which is in an intermediate state of theplurality of pieces of image data is output.

The first memory 7621 may have a structure in which the external imagesignal 7600 is input, the external image signal 7600 is held for acertain period, and the external image signal 7600 is output to themotion detection circuit 7620 and the second memory 7622.

The second memory 7622 may have a structure in which image data outputfrom the first memory 7621 is input, the image data is held for acertain period, and the image data is output to the motion detectioncircuit 7620 and the high-speed processing circuit 7625.

The third memory 7623 may have a structure in which image data outputfrom the motion detection circuit 7620 is input, the image data is heldfor a certain period, and the image data is output to the luminancecontrol circuit 7624.

The high-speed processing circuit 7625 may have a structure in whichimage data output from the second memory 7622, image data output fromthe luminance control circuit 7624, and a frequency control signal 7608are input and the image data is output as the image signal 7603.

In the case where the frequency of the external image signal 7600 isdifferent from the frequency of the image signal 7603, the image signal7603 may be generated by interpolating the image signal included in theexternal image signal 7600 by the image processing circuit 7615. Theinput external image signal. 7600 is once held in the first memory 7621.At that time, image data which is input in the previous frame is held inthe second memory 7622. The motion detection circuit 7620 may read theimage data held in the first memory 7621 and the second memory 7622 asappropriate to detect a motion vector by difference between the bothpieces of image data and to generate image data in an intermediatestate. The generated image data in an intermediate state is held in thethird memory 7623.

When the motion detection circuit 7620 generates the image data in anintermediate state, the high-speed processing circuit 7625 outputs theimage data held in the second memory 7622 as the image signal 7603.After that, the image data held in the third memory 7623 is outputthrough the luminance control circuit 7624 as the image signal 7603. Atthis time, frequency which is updated by the second memory 7622 and thethird memory 7623 is the same as the external image signal 7600;however, the frequency of the image signal 7603 which is output throughthe high-speed processing circuit 7625 may be different from thefrequency of the external image signal 7600. Specifically, for example,the frequency of the image signal 7603 is 1.5 times, twice, or threetimes as high as the frequency of the external image signal 7600.However, the present invention is not limited to this, and a variety offrequency can be used. Note that the frequency of the image signal 7603may be specified by the frequency control signal 7608.

The structure of the image processing circuit 7615 shown in FIG. 76D isobtained by adding a fourth memory 7626 to the structure of the imageprocessing circuit 7615 shown in FIG. 76C. When image data output fromthe fourth memory 7626 is also output to the motion detection circuit7620 in addition to the image data output from the first memory 7621 andthe image data output from the second memory 7622 in this manner,movement of an image can be detected adequately.

Note that in the case where image data to be input has already includeda motion vector for data compression or the like, for example, the imagedata to be input is image data which is based on an MPEG (moving pictureexpert group) standard, an image in an intermediate state may begenerated as an interpolated image by using this image data. At thistime, a portion which generates a motion vector included in the motiondetection circuit 7620 is not necessary. Further, since encoding anddecoding processing of the image signal 7603 is simplified, powerconsumption can be reduced.

Note that although this embodiment mode is described with reference tovarious drawings, the contents (or may be part of the contents)described in each drawing can be freely applied to, combined with, orreplaced with the contents (or may be part of the contents) described inanother drawing. Further, even more drawings can be formed by combiningeach part with another part in the above-described drawings.

Similarly, the contents (or may be part of the contents) described ineach drawing of this embodiment mode can be freely applied to, combinedwith, or replaced with the contents (or may be part of the contents)described in a drawing in another embodiment mode. Further, even moredrawings can be formed by combining each part with part of anotherembodiment mode in the drawings of this embodiment mode.

Note that this embodiment mode shows an example of an embodied case ofthe contents (or may be part of the contents) described in otherembodiment modes, an example of slight transformation thereof, anexample of partial modification thereof, an example of improvementthereof, an example of detailed description thereof, an applicationexample thereof, an example of related part thereof, or the like.Therefore, the contents described in other embodiment modes can befreely applied to, combined with, or replaced with this embodiment mode.

[Embodiment Mode 11]

In this embodiment mode, a structure and a manufacturing method of atransistor are described.

FIGS. 48A to 48G show examples of structures and manufacturing methodsof transistors included in a display device of the present invention.FIG. 48A shows structure examples of transistors included in the displaydevice of the present invention. FIGS. 48B to 48G show examples ofmanufacturing methods of the transistors included in the display deviceof the present invention.

Note that the structure and the manufacturing method of the transistorsincluded in the display device of the present invention are not limitedto those shown in FIGS. 48A to 48G, and various structures andmanufacturing methods can be employed.

First, structure examples of transistors included in the display deviceof the present invention are described with reference to FIG. 48A. FIG.48A is a cross-sectional view of a plurality of transistors each havinga different structure. Here, in FIG. 48A, the plurality of transistorseach having a different structure are juxtaposed, which is fordescribing structures of the transistors. Thus, the transistors are notneeded to be actually juxtaposed as shown in FIG. 48A and can beseparately formed as needed.

Next, characteristics of each layer forming the transistor included inthe display device of the present invention are described.

A substrate 4011 can be a glass substrate using barium borosilicateglass, aluminoborosilicate glass, or the like, a quartz substrate, aceramic substrate, a metal substrate containing stainless steel, or thelike. Further, a substrate formed of plastics typified by polyethyleneterephthalate (PET), polyethylene naphthalate (PEN), or polyethersulfone(PES), or a substrate formed of a flexible synthetic resin such asacrylic can also be used. By using a flexible substrate, a displaydevice capable of being bent can be formed. A flexible substrate has nostrict limitations on the area or the shape of the substrate.

An insulating film 4012 functions as a base film and is provided toprevent alkali metal such as Na or alkaline earth metal from thesubstrate 4011 from adversely affecting characteristics of asemiconductor element. The insulating film 4012 can have a single-layerstructure or a stacked-layer structure of an insulating film containingoxygen or nitrogen, such as silicon oxide (SiO_(x)), silicon nitride(SiN_(x)), silicon oxynitride (SiO_(x)N_(y)) (x>y), or silicon nitrideoxide (SiN_(x)O_(y))(x>y). For example, when the insulating film 4012 isprovided to have a two-layer structure, it is preferable that a siliconnitride oxide film be used as a first insulating film and a siliconoxynitride film be used as a second insulating film. Further, when theinsulating film 4012 is provided to have a three-layer structure, it ispreferable that a silicon oxynitride film be used as a first insulatingfilm, a silicon nitride oxide film be used as a second insulating film,and a silicon oxynitride film be used as a third insulating film.

Semiconductor layers 4013, 4014, and 4015 can be formed using anamorphous semiconductor or a semi-amorphous semiconductor (SAS).Alternatively, a polycrystalline semiconductor layer may be used. SAS isa semiconductor having an intermediate structure between amorphous andcrystalline (including single crystal and polycrystalline) structuresand having a third state which is stable in free energy.

Moreover, SAS includes a crystalline region with a short-range order andlattice distortion. A crystalline region of 0.5 to 20 nm can be observedat least in part of a film. When silicon is contained as a maincomponent, Raman spectrum shifts to a wave number side lower than 520cm⁻¹. The diffraction peaks of (111) and (220) which are thought to becontributed to a silicon crystalline lattice are observed by X-raydiffraction. SAS contains hydrogen or halogen of at least 1 atomicpercent or more to compensate dangling bonds. SAS is formed by glowdischarge decomposition (plasma CVD) of a material gas. As the materialgas, Si₂H₆, SiH₂Cl₂, SiHCl₃, SiCl₄, SiF₄, or the like as well as SiH₄can be used. Alternatively, GeF₄ may be mixed. The material gas may bediluted with H₂, or H₂ and one or more kinds of rare gas elementsselected from He, Ar, Kr, and Ne. A dilution ratio is in the range of 2to 1000 times. Pressure is in the range of approximately 0.1 to 133 Pa,and a power supply frequency is 1 to 120 MHz, preferably 13 to 60 MHz. Asubstrate heating temperature may be 300° C. or lower. A concentrationof impurities in atmospheric components such as oxygen, nitrogen, andcarbon is preferably 1×10²⁰ cm⁻¹ or less as impurity elements in thefilm. In particular, an oxygen concentration is 5×10¹⁹/cm³ or less,preferably 1×10¹⁹/cm³ or less. Here, an amorphous semiconductor layer isformed using a material containing silicon (Si) as its main component(e.g., Si_(x)Ge_(1-x)) by a known method (such as a sputtering method,an LPCVD method, or a plasma CVD method). Then, the amorphoussemiconductor layer is crystallized by a known crystallization methodsuch as a laser crystallization method, a thermal crystallization methodusing RTA or an annealing furnace, or a thermal crystallization methodusing a metal element which promotes crystallization.

An insulating film 4016 can have a single-layer structure or astacked-layer structure of an insulating film containing oxygen ornitrogen, such as silicon oxide (SiO_(x)), silicon nitride (SiN_(x)),silicon oxynitride (SiO_(x)N_(y)) (x>y), or silicon nitride oxide(SiN_(x)O_(y)) (x>y).

A gate electrode 4017 can have a single-layer structure of a conductivefilm or a stacked-layer structure of two or three conductive films. As amaterial for the gate electrode 4017, a known conductive film can beused. For example, a single film of an element such as tantalum (Ta),titanium (Ti), molybdenum (Mo), tungsten (W), chromium (Cr), or silicon(Si); a nitride film containing the aforementioned element (typically, atantalum nitride film, a tungsten nitride film, or a titanium nitridefilm); an alloy film in which the aforementioned elements are combined(typically, a Mo—W alloy or a Mo—Ta alloy); a silicide film containingthe aforementioned element (typically, a tungsten silicide film or atitanium silicide film); and the like can be used. Note that theaforementioned single film, nitride film, alloy film, silicide film, andthe like can have a single-layer structure or a stacked-layer structure.

An insulating film 4018 can have a single-layer structure or astacked-layer structure of an insulating film containing oxygen ornitrogen, such as silicon oxide (SiO_(x)), silicon nitride (SiN_(x)),silicon oxynitride (SiO_(x)N_(y)) (x>y), or silicon nitride oxide(SiN_(x)O_(y)) (x>y); or a film containing carbon, such as a DLC(diamond-like carbon), by a known method (such as a sputtering method ora plasma CVD method).

An insulating film 4019 can have a single-layer structure or astacked-layer structure of a siloxane resin; an insulating filmcontaining oxygen or nitrogen, such as silicon oxide (SiO_(x)), siliconnitride (SiN_(x)), silicon oxynitride (SiO_(x)N_(y)) (x>y), or siliconnitride oxide (SiN_(x)O_(y)) (x>y); a film containing carbon, such as aDLC (diamond-like carbon); or an organic material such as epoxy,polyimide, polyamide, polyvinyl phenol, benzocyclobutene, or acrylic.Note that a siloxane resin corresponds to a resin having Si—O—Si bonds.Siloxane includes a skeleton structure of a bond of silicon (Si) andoxygen (O). As a substituent, an organic group containing at leasthydrogen (such as an alkyl group or aryl group) is used. Alternatively,a fluoro group, or a fluoro group and an organic group containing atleast hydrogen can be used as a substituent. Note that in a displaydevice applicable to the present invention, the insulating film 4019 canbe directly provided so as to cover the gate electrode 4017 withoutprovision of the insulating film 4018.

As a conductive film 4023, a single film of an element such as Al, Ni,W, Mo, Ti, Pt, Cu, Ta, Au, or Mn, a nitride film containing theaforementioned element, an alloy film in which the aforementionedelements are combined, a silicide film containing the aforementionedelement, or the like can be used. For example, as an alloy containing aplurality of the aforementioned elements, an Al alloy containing C andTi, an Al alloy containing Ni, an Al alloy containing C and Ni, an Alalloy containing C and Mn, or the like can be used. Further, when theconductive film has a stacked-layer structure, Al can be interposedbetween Mo, Ti, or the like; thus, resistance of Al to heat and chemicalreaction can be improved.

Next, with reference to the cross-sectional view of the plurality oftransistors each having a different structure shown in FIG. 48A,characteristics of each structure are described.

A transistor 4001 is a single drain transistor. Since the single draintransistor can be formed by a simple method, it is advantageous in lowmanufacturing cost and high yield. Here, the semiconductor layers 4013and 4015 have different concentrations of impurities, and thesemiconductor layer 4013 is used as a channel formation region and thesemiconductor layers 4015 serve as a source region and a drain region.By controlling the concentration of impurities in this manner,resistivity of the semiconductor layer can be controlled. Further, anelectrical connection state of the semiconductor layer and theconductive film 4023 can be closer to ohmic contact. Note that as amethod of separately forming the semiconductor layers each havingdifferent amount of impurities, a method can be used in which impuritiesare doped in a semiconductor layer using the gate electrode 4017 as amask.

A transistor 4002 is a transistor in which the gate electrode 4017 istapered at an angle of at least certain degrees. Since the transistorcan be formed by a simple method, it is advantageous in lowmanufacturing cost and high yield. The semiconductor layers 4013, 4014,and 4015 have different concentrations of impurities. The semiconductorlayer 4013 is used as a channel formation region, the semiconductorlayers 4014 as lightly doped drain (LDD) regions, and the semiconductorlayers 4015 as a source region and a drain region. By controlling theamount of impurities in this manner, resistivity of the semiconductorlayer can be controlled. Further, an electrical connection state of thesemiconductor layer and the conductive film 4023 can be closer to ohmiccontact. Moreover, since the transistor includes the LDD regions, a highelectric field is hardly applied inside the transistor, so thatdeterioration of the element due to hot carriers can be suppressed. Notethat as a method of separately forming the semiconductor layers havingdifferent amount of impurities, a method can be used in which impuritiesare doped in a semiconductor layer using the gate electrode 4017 as amask. In the transistor 4002, since the gate electrode 4017 is taperedat an angle of at least certain degrees, gradient of the concentrationof, impurities doped in the semiconductor layer through the gateelectrode 4017 can be provided, and the LDD region can be easily formed.

A transistor 4003 is a transistor in which the gate electrode 4017 isformed of at least two layers, and a lower gate electrode 4017 a islonger than an upper gate electrode 4017 b. In this specification, ashape of the lower and upper gate electrodes is called a hat shape. Whenthe gate electrode has a hat shape, an LDD region can be formed withoutaddition of a photomask. Note that a structure where the LDD regionoverlaps with the gate electrode, like the transistor 4003, isparticularly called a GOLD (gate overlapped LDD) structure. As a methodof forming the gate electrode with a hat shape, the following method maybe used.

First, when the gate electrode is patterned, the lower and upper gateelectrodes are etched by dry etching so that side surfaces thereof areinclined (tapered). Then, the inclination of the upper gate electrode isprocessed to be almost perpendicular by anisotropic etching. Thus, thegate electrode a cross section of which is a hat shape is formed. Afterthat, impurity elements are doped twice, so that the semiconductor layer4013 used as the channel region, the semiconductor layers 4014 used asthe LDD regions, and the semiconductor layers 4015 used as a sourceelectrode and a drain electrode are formed.

Note that here, part of the LDD region, which overlaps with the gateelectrode, is referred to as an Lov region, and part of the LDD region,which does not overlap with the gate electrode, is referred to as anLoff region. The Loff region is highly effective in suppressing anoff-current value, whereas it is not very effective in preventingdeterioration in an on-current value due to hot carriers by relieving anelectric field in the vicinity of the drain. On the other hand, the Lovregion is effective in preventing deterioration in the on-current valueby relieving the electric field in the vicinity of the drain, whereas itis not very effective in suppressing the off-current value. Thus, it ispreferable to form a transistor having a structure appropriate forcharacteristics of each of the various circuits. For example, when adisplay device to which the present invention can be applied is used, atransistor having an Loff region is preferably used as a pixeltransistor in order to suppress the off-current value. On the otherhand, as a transistor in a peripheral circuit, a transistor having anLov region is preferably used in order to prevent deterioration in theon-current value by relieving the electric field in the vicinity of thedrain.

A transistor 4004 is a transistor including a sidewall 4021 in contactwith the side surface of the gate electrode 4017. When the transistorincludes the sidewall 4021, a region overlapping with the sidewall 4021can be made to be an LDD region.

A transistor 4005 is a transistor in which an LDD (Loff) region isformed by performing doping of the semiconductor layer with the use of amask. Thus, the LDD region can surely be formed, and an off-currentvalue of the transistor can be reduced.

A transistor 4006 is a transistor in which an LDD (Lov) region is formedby performing doping of the semiconductor layer with the use of a maskThus, the LDD region can surely be formed, and deterioration in anon-current value can be prevented by relieving the electric field in thevicinity of the drain of the transistor.

Next, an example of a method for manufacturing a transistor included inthe display device to which the present invention can be applied isdescribed with reference to FIGS. 48B to 48G.

Note that a structure and a manufacturing method of a transistorincluded in the display device of the present invention is not limitedto those in FIGS. 48A to 48G, and various structures and manufacturingmethods can be used.

In this embodiment mode, surfaces of the substrate 4011, the insulatingfilm 4012, the semiconductor layer 4013, the semiconductor layer 4014,the semiconductor layer 4015, the insulating film 4016, the insulatingfilm 4018, or the insulating film 4019 are oxidized or nitrided by usingplasma treatment, so that these surfaces can be oxidized or nitrided. Byoxidizing or nitriding the semiconductor layer or the insulating film byplasma treatment in such a manner, the surface of the semiconductorlayer or the insulating film is modified, and the insulating film can beformed to be denser than an insulating film formed by a CVD method or asputtering method. Thus, a defect such as a pinhole can be suppressed,and characteristics and the like of the display device can be improved.

First, the surface of the substrate 4011 is washed using hydrofluoricacid (HF), alkaline, or pure water. The substrate 4011 can be a glasssubstrate using barium borosilicate glass, aluminoborosilicate glass, orthe like, a quartz substrate, a ceramic substrate, a metal substratecontaining stainless steel, or the like. Further, a substrate formed ofplastics typified by polyethylene terephthalate (PET), polyethylenenaphthalate (PEN), or polyethersulfone (PES), or a substrate formed of aflexible synthetic resin such as acrylic can also be used. Here, thecase where a glass substrate is used as the substrate 4011 is shown.

Here, an oxide film or a nitride film may be formed on the surface ofthe substrate 4011 by oxidizing or nitriding the surface of thesubstrate 4011 by plasma treatment (FIG. 48B). Hereinafter, aninsulating film such as an oxide film or a nitride film formed byperforming plasma treatment on the surface is also referred to as aplasma-treated insulating film. Note that in FIG. 48B, an insulatingfilm 4031 is a plasma-treated insulating film. In general, when asemiconductor element such as a thin film transistor is provided over asubstrate formed of glass, plastic, or the like, an impurity elementsuch as alkali metal (e.g., Na) or alkaline earth metal included inglass, plastic, or the like might be mixed into the semiconductorelement; thus, characteristics of the semiconductor element may beadversely affected in some cases. Nitridation of a surface of thesubstrate formed of glass, plastic, or the like can prevent an impurityelement such as alkali metal (e.g., Na) or alkaline earth metal includedin the substrate from being mixed into the semiconductor element.

When the surface is oxidized by plasma treatment, the plasma treatmentis performed in an oxygen atmosphere (e.g., in an atmosphere of oxygen(O₂) and a rare gas (containing at least one of He, Ne, Ar, Kr, and Xe),in an atmosphere of oxygen, hydrogen (H₂), and a rare gas, or in anatmosphere of dinitrogen monoxide and a rare gas). On the other hand,when the semiconductor layer is nitrided by plasma treatment, the plasmatreatment is performed in a nitrogen atmosphere (e.g., in an atmosphereof nitrogen (N₂) and a rare gas (containing at least one of He, Ne, Ar,Kr, and Xe), in an atmosphere of nitrogen, hydrogen, and a rare gas, orin an atmosphere of NH₃ and a rare gas). As a rare gas, for example, Aror a gas in which Ar and Kr are mixed may be used. Accordingly, theplasma-treated insulating film contains a rare gas (containing at leastone of He, Ne, Ar, Kr, and Xe) used for the plasma treatment. Forexample, the plasma-treated insulating film contains Ar when Ar is used.

In addition, it is preferable to process plasma treatment in theatmosphere containing the aforementioned gas, with conditions of anelectron density in the range of 1×10¹¹ to 1×10¹³ cm⁻³ and a plasmaelectron temperature in the range of 0.5 to 1.5 eV. Since the plasmaelectron density is high and the electron temperature in the vicinity ofan object to be treated is low, damage by plasma to the object to betreated can be prevented. Further, since the plasma electron density isas high as 1×10¹¹ cm⁻³ or more, an oxide film or a nitride film formedby oxidizing or nitriding the object to be treated by plasma treatmentis superior in its uniformity of thickness and the like as well as beingdense, as compared with a film formed by a CVD method, a sputteringmethod, or the like. Alternatively, since the plasma electrontemperature is as low as 1 eV or less, oxidation or nitridation can beperformed at a lower temperature as compared with a conventional plasmatreatment or thermal oxidation. For example, oxidation or nitridationcan be performed sufficiently even when plasma treatment is performed ata temperature lower than a strain point of a glass substrate by 100degrees or more. Note that as frequency for generating plasma, highfrequency waves such as microwaves (2.45 GHz) can be used. Note thathereinafter, plasma treatment is performed using the aforementionedconditions unless otherwise specified.

Note that although FIG. 48B shows the case where the plasma-treatedinsulating film is formed by plasma treatment on the surface of thesubstrate 4011, this embodiment mode includes the case where aplasma-treated insulating film is not formed on the surface of thesubstrate 4011.

Note that although a plasma-treated insulating film formed by plasmatreatment on the surface of the object to be treated is not shown inFIGS. 48C to 48G this embodiment mode includes the case where aplasma-treated insulating film formed by plasma treatment exists on thesurface of the substrate 4011, the insulating film 4012, thesemiconductor layer 4013, the semiconductor layer 4014, thesemiconductor layer 4015, the insulating film 4016, the insulating film4018, or the insulating film 4019.

Next, the insulating film 4012 is formed over the substrate 4011 by aknown method (such as a sputtering method, an LPCVD method, or a plasmaCVD method) (FIG. 48C). For the insulating film 4012, silicon oxide(SiO_(x)) or silicon oxynitride (SiO_(x)N_(y)) (x>y) can be used.

Here, a plasma-treated insulating film may be formed on the surface ofthe insulating film 4012 by oxidizing or nitriding the surface of theinsulating film 4012 by plasma treatment. By oxidizing the surface ofthe insulating film 4012, the surface of the insulating film 4012 ismodified, and a dense film with fewer defects such as a pinhole can beobtained. Further, by oxidizing the surface of the insulating film 4012,the plasma-treated insulating film containing a little amount of N atomscan be formed; thus, interface characteristics of the plasma-treatedinsulating film and a semiconductor layer are improved when thesemiconductor layer is provided over the plasma-treated insulating film.The plasma-treated insulating film contains a rare gas (containing atleast one of He, Ne, Ar, Kr, and Xe) used for the plasma treatment. Notethat the plasma treatment can be similarly performed under theaforementioned conditions.

Next, the island-shaped semiconductor layers 4013 and 4014 are formedover the insulating film 4012 (FIG. 48D). The island-shapedsemiconductor layers 4013 and 4014 can be formed in such a manner thatan amorphous semiconductor layer is formed over the insulating film 4012by using a material containing silicon (Si) as its main component (e.g.,Si_(x)Ge_(1-x)) or the like by a known method (such as a sputteringmethod, an LPCVD method, or a plasma CVD method), the amorphoussemiconductor layer is crystallized, and the semiconductor layer isselectively etched. Note that crystallization of the amorphoussemiconductor layer can be performed by a known crystallization methodsuch as a laser crystallization method, a thermal crystallization methodusing RTA or an annealing furnace, a thermal crystallization methodusing a metal element which promotes crystallization, or a method inwhich these methods are combined. Here, end portions of theisland-shaped semiconductor layers are provided with an angle of about90° (θ=85 to 100°). Alternatively, the semiconductor layer 4014 to be alow concentration drain region may be formed by doping impurities withthe use of a mask.

Here, a plasma-treated insulating film may be formed on the surfaces ofthe semiconductor layers 4013 and 4014 by oxidizing or nitriding thesurfaces of the semiconductor layers 4013 and 4014 by plasma treatment.For example, when Si is used for the semiconductor layers 4013 and 4014,silicon oxide (SiO_(x)) or silicon nitride (SiN_(x)) is formed as theplasma-treated insulating film. Alternatively, after being oxidized byplasma treatment, the semiconductor layers 4013 and 4014 may be nitridedby performing plasma treatment again. In this case, silicon oxide(SiO_(x)) is formed in contact with the semiconductor layers 4013 and4014, and silicon nitride oxide (SiN_(x)O_(y)) (x>y) is formed on thesurface of the silicon oxide. Note that when the semiconductor layer isoxidized by plasma treatment, the plasma treatment is performed in anoxygen atmosphere (e.g., in an atmosphere of oxygen (O₂) and a rare gas(containing at least one of He, Ne, Ar, Kr, and Xe), in an atmosphere ofoxygen, hydrogen (H₂), and a rare gas, or in an atmosphere of dinitrogenmonoxide and a rare gas). On the other hand, when the semiconductorlayer is nitrided by plasma treatment, the plasma treatment is performedin a nitrogen atmosphere (e.g., in an atmosphere of nitrogen (N₂) and arare gas (containing at least one of He, Ne, Ar, Kr, and Xe), in anatmosphere of nitrogen, hydrogen, and a rare gas, or in an atmosphere ofNH₃ and a rare gas). As a rare gas, Ar can be used, for example.Alternatively, a gas in which Ar and Kr are mixed may be used.Accordingly, the plasma-treated insulating film contains a rare gas(containing at least one of He, Ne, Ar, Kr, and Xe) used for the plasmatreatment. For example, the plasma-treated insulating film contains Arwhen Ar is used.

Next, the insulating film 4016 is formed (FIG. 48E). The insulating film4016 can have a single-layer structure or a stacked-layer structure ofan insulating film containing oxygen or nitrogen, such as silicon oxide(SiO_(x)), silicon nitride (SiN_(x)), silicon oxynitride (SiO_(x)N_(y))(x>y), or silicon nitride oxide (SiN_(x)O_(y)) (x>y), by a known method(such as a sputtering method, an LPCVD method, or a plasma CVD method).Note that when the plasma-treated insulating film is formed on thesurfaces of the semiconductor layers 4013 and 4014 by performing plasmatreatment on the surfaces of the semiconductor layers 4013 and 4014, theplasma-treated insulating film can be used as the insulating film 4016.

Here, the surface of the insulating film 4016 may be oxidized ornitrided by plasma treatment, so that a plasma-treated insulating filmis formed on the surface of the insulating film 4016. Note that theplasma-treated insulating film contains a rare gas (containing at leastone of He, Ne, Ar, Kr, and Xe) used for the plasma treatment. The plasmatreatment can be similarly performed under the aforementionedconditions.

Alternatively, after the insulating film 4016 is oxidized by performingplasma treatment once in an oxygen atmosphere, the insulating film 4016may be nitrided by performing plasma treatment again in a nitrogenatmosphere. By oxidizing or nitriding the surface of the insulating film4016 by plasma treatment in such a manner, the surface of the insulatingfilm 4016 is modified, and a dense film can be formed. An insulatingfilm obtained by plasma treatment is denser and has fewer defects suchas a pinhole, as compared with an insulating film formed by a CVDmethod, a sputtering method, or the like. Thus, characteristics of athin film transistor can be improved.

Next, the gate electrode 4017 is formed (FIG. 48F). The gate electrode4017 can be formed by a known method (such as a sputtering method, anLPCVD method, or a plasma CVD method).

In the transistor 4001, the semiconductor layers 4015 used as the sourceregion and the drain region can be formed by doping impurities after thegate electrode 4017 is formed.

In the transistor 4002, the semiconductor layers 4014 used as the LDDregions, the semiconductor layer 4013, and the semiconductor layers 4015used as the source region and the drain region can be formed by dopingimpurities after the gate electrode 4017 is formed.

In the transistor 4003, the semiconductor layers 4014 used as the LDDregions, the semiconductor layer 4013, and the semiconductor layers 4015used as the source region and the drain region can be formed by dopingimpurities after the gate electrode 4017 a and 4017 b is formed.

In the transistor 4004, the semiconductor layers 4014 used as the LDDregions, the semiconductor layer 4013, and the semiconductor layers 4015used as the source region and the drain region can be formed by dopingimpurities after the sidewall 4021 is formed on the side surface of thegate electrode 4017.

Note that silicon oxide (SiO_(x)) or silicon nitride (SiN_(x)) can beused for the sidewall 4021. As a method of forming the sidewall 4021 onthe side surface of the gate electrode 4017, a method can be used, forexample, in which a silicon oxide (SiO_(x)) film or a silicon nitride(SiN_(x)) film is formed by a known method after the gate electrode 4017is formed, and then, the silicon oxide (SiO_(x)) film or the siliconnitride (SiN_(x)) film is etched by anisotropic etching. Thus, thesilicon oxide (SiO_(x)) film or the silicon nitride (SiN_(x)) filmremains only on the side surface of the gate electrode 4017, so that thesidewall 4021 can be formed on the side surface of the gate electrode4017.

In the transistor 4005, the semiconductor layers 4014 used as the LDD(Loll) regions, the semiconductor layers 4013, and the semiconductorlayer 4015 used as the source region and the drain region can be formedby doping impurities after a mask 4022 is formed to cover the gateelectrode 4017.

In the transistor 4006, the semiconductor layers 4014 used as the LDD(Lov) regions, the semiconductor layers 4013, and the semiconductorlayers 4015 used as the source region and the drain region can be formedby doping impurities after the gate electrode 4017 is formed.

Next, the insulating film 4018 is formed (FIG. 48G). The insulating film4018 can have a single-layer structure or a stacked-layer structure ofan insulating film containing oxygen or nitrogen, such as silicon oxide(SiO_(x)), silicon nitride (SiN_(x)), silicon oxynitride (SiO_(x)N_(y))(x>y), or silicon nitride oxide (SiN_(x)O_(y)) (x>y); or a filmcontaining carbon, such as a DLC (diamond-like carbon), by a knownmethod (such as a sputtering method or a plasma CVD method).

Here, the surface of the insulating film 4018 may be oxidized ornitrided by plasma treatment, so that a plasma-treated insulating filmis formed on the surface of the insulating film 4018. Note that theplasma-treated insulating film contains a rare gas (containing at leastone of He, Ne, Ar, Kr, and Xe) used for the plasma treatment. The plasmatreatment can be similarly performed under the aforementionedconditions.

Next, the insulating film 4019 is formed (FIG. 48A). The insulating film4019 can have a single-layer structure or a stacked-layer structure ofan organic material such as epoxy, polyimide, polyamide, polyvinylphenol, benzocyclobutene, or acrylic; or a siloxane resin, in additionto an insulating film containing oxygen or nitrogen, such as siliconoxide (SiO_(x)), silicon nitride (SiN_(x)), silicon oxynitride(SiO_(x)N_(y)) (x>y), or silicon nitride oxide (SiN_(x)O_(y)) (x>y); ora film containing carbon, such as a DLC (diamond-like carbon), by knownmethod (such as a sputtering method or a plasma CVD method). Note that asiloxane resin corresponds to a resin having Si—O—Si bonds. Siloxaneincludes a skeleton structure of a bond of silicon (Si) and oxygen (O).As a substituent, an organic group containing at least hydrogen (such asan alkyl group or aryl group) is used. Alternatively, a fluoro group, ora fluoro group and an organic group containing at least hydrogen can beused as a substituent. In addition, the plasma-treated insulating filmcontains a rare gas (containing at least one of He, Ne, Ar, Kr, and Xe)used for the plasma treatment. For example, the plasma-treatedinsulating film contains Ar when Ar is used.

When an organic material such as polyimide, polyamide, polyvinyl phenol,benzocyclobutene, or acrylic, or a siloxane resin is used for theinsulating film 4019, the surface of the insulating film 4019 can bemodified by oxidizing or nitriding the surface of the insulating film byplasma treatment. Modification of the surface improves strength of theinsulating film 4019, and physical damage such as a crack generated whenan opening is formed, for example, or film reduction in etching can bereduced. Further, when the conductive film 4023 is formed over theinsulating film 4019, modification of the surface of the insulating film4019 improves adhesion to the conductive film. For example, when asiloxane resin is used for the insulating film 4019 and nitrided byplasma treatment, a plasma-treated insulating film containing nitrogenor a rare gas is formed by nitriding a surface of the siloxane resin,and physical strength is improved.

Next, a contact hole is formed in the insulating films 4019, 4018, and4016 in order to form the conductive film 4023 which is electricallyconnected to the semiconductor layer 4015. Note that the contact holemay have a tapered shape; thus, coverage with the conductive film 4023can be improved.

FIG. 49 shows cross-sectional structures of a bottom-gate transistor anda capacitor.

A first insulating film (an insulating film 4102) is formed over anentire surface of a substrate 4101. The first insulating film canprevent impurities from the substrate from adversely affecting asemiconductor layer and changing properties of a transistor. That is,the first insulating film functions as a base film. Thus, a transistorwith high reliability can be formed. As the first insulating film, asingle layer or a stacked layer of a silicon oxide film, a siliconnitride film, a silicon oxynitride film (SiO_(x)N_(y)), or the like canbe used.

A first conductive layer (conductive layers 4103 and 4104) is formedover the first insulating film. The conductive layer 4103 includes aportion functioning as a gate electrode of a transistor 4120. Theconductive layer 4104 includes a portion functioning as a firstelectrode of a capacitor 4121. As the first conductive layer, an elementsuch as Ti, Mo, Ta, Cr, W, Al, Nd, Cu, Ag, Au, Pt, Si, Zn, Fe, Ba, orGe, or an alloy of these elements can be used. Alternatively, a stackedlayer of these elements (including the alloy thereof) can be used.

A second insulating film (an insulating film 4122) is formed to cover atleast the first conductive layer. The second insulating film functionsas a gate insulating film. As the second insulating film, a single layeror a stacked layer of a silicon oxide film, a silicon nitride film, asilicon oxynitride film (SiO_(x)N_(y)), or the like can be used.

Note that for a portion of the second insulating film, which is incontact with the semiconductor layer, a silicon oxide film is preferablyused. This is because the trap level at the interface between thesemiconductor layer and the second insulating film is lowered.

When the second insulating film is in contact with Mo, a silicon oxidefilm is preferably used for a portion of the second insulating film incontact with Mo. This is because the silicon oxide film does not oxidizeMo.

A semiconductor layer is formed in part of a portion over the secondinsulating film, which overlaps with the first conductive layer, by aphotolithography method, an inkjet method, a printing method, or thelike. Part of the semiconductor layer extends to a portion over thesecond insulating film, which does not overlap with the first conductivelayer. The semiconductor layer includes a channel formation region (achannel formation region 4110), an LDD region (LDD regions 4108 and4109), and an impurity region (impurity regions 4105, 4106, and 4107).The channel formation region 4110 functions as a channel formationregion of the transistor 4120. The LDD regions 4108 and 4109 function asLDD regions of the transistor 4120. Note that the LDD regions 4108 and4109 are not necessarily formed. The impurity region 4105 includes aportion functioning as one of a source region and a drain region of thetransistor 4120. The impurity region 4106 includes a portion functioningas the other of the source region and the drain region of the transistor4120. The impurity region 4107 includes a portion functioning as asecond electrode of the capacitor 4121.

A third insulating film (an insulating film 4111) is entirely formed. Acontact hole is selectively formed in part of the third insulating film.The insulating film 4111 functions as an interlayer film. As the thirdinsulating film, an inorganic material (e.g., silicon oxide, siliconnitride, or silicon oxynitride), an organic compound material having alow dielectric constant (e.g., a photosensitive or nonphotosensitiveorganic resin material), or the like can be used. Alternatively, amaterial containing siloxane may be used. Note that siloxane is amaterial in which a skeleton structure is formed by a bond of silicon(Si) and oxygen (O). As a substitute, an organic group containing atleast hydrogen (such as an alkyl group or aryl group) is used.Alternatively, a fluoro group, or a fluoro group and an organic groupcontaining at least hydrogen may be used as a substituent.

A second conductive layer (conductive layers 4112 and 4113) is formedover the third insulating film. The conductive layer 4112 is connectedto the other of the source electrode and the drain electrode of thetransistor 4120 through the contact hole formed in the third insulatingfilm. Thus, the conductive layer 4112 includes a portion functioning asthe other of the source region and the drain region of the transistor4120. As the second conductive layer, an element such as Ti, Mo, Ta, Cr,W, Al, Nd, Cu, Ag, Au, Pt, Si, Zn, Fe, Ba, or Ge, or an alloy of theseelements can be used. Alternatively, a stacked layer of these elements(including the alloy thereof) can be used.

Note that in steps after forming the second conductive layer, variousinsulating films or various conductive films may be formed.

Next, structures of a transistor and a capacitor are described in thecase where an amorphous silicon (a-Si:H) film is used as a semiconductorlayer of the transistor.

FIG. 50 shows cross-sectional structures of a top-gate transistor and acapacitor.

A first insulating film (an insulating film 4202) is formed over anentire surface of a substrate 4201. The first insulating film canprevent impurities from the substrate from adversely affecting asemiconductor layer and changing properties of a transistor. That is,the first insulating film functions as a base film Thus, a transistorwith high reliability can be formed. As the first insulating film, asingle layer or a stacked layer of a silicon oxide film, a siliconnitride film, a silicon oxynitride film (SiO_(x)N_(y)), or the like canbe used.

Note that the first insulating film is not necessarily formed. When thefirst insulating film is not formed, reduction in the number of stepsand reduction in manufacturing cost can be realized. Further, since thestructure can be simplified, the yield can be improved.

A first conductive layer (conductive layers 4203, 4204, and 4205) isformed over the first insulating film. The conductive layer 4203includes a portion functioning as one of a source electrode and a drainelectrode of a transistor 4220. The conductive layer 4204 includes aportion functioning as the other of the source electrode and the drainelectrode of the transistor 4220. The conductive layer 4205 includes aportion functioning as a first electrode of a capacitor 4221. As thefirst conductive layer, an element such as Ti, Mo, Ta, Cr, W, Al, Nd,Cu, Ag, Au, Pt, Si, Zn, Fe, Ba, or Ge, or an alloy of these elements canbe used. Alternatively, a stacked layer of these elements (including thealloy thereof) can be used.

A first semiconductor layer (semiconductor layers 4206 and 4207) isformed above the conductive layers 4203 and 4204. The semiconductorlayer 4206 includes a portion functioning as one of the source regionthe drain region. The semiconductor layer 4207 includes a portionfunctioning as the other of the source electrode and the drainelectrode. As the first semiconductor layer, silicon containingphosphorus or the like can be used, for example.

A second semiconductor layer (a semiconductor layer 4208) is formed overthe first insulating film and between the conductive layer 4203 and theconductive layer 4204. Part of the semiconductor layer 4208 extends overthe conductive layers 4203 and 4204. The semiconductor layer 4208includes a portion functioning as a channel region of the transistor4220. As the second semiconductor layer, a semiconductor layer having nocrystallinity such as an amorphous silicon (a-Si:H) layer, asemiconductor layer such as a microcrystalline semiconductor (μ-Si:H)layer, or the like can be used.

A second insulating film (insulating films 4209 and 4210) is formed tocover at least the semiconductor layer 4208 and the conductive layer4205. The second insulating film functions as a gate insulating film. Asthe second insulating film, a single layer or a stacked layer of asilicon oxide film, a silicon nitride film, a silicon oxynitride film(SiO_(x)N_(y)), or the like can be used.

Note that for a portion of the second insulating film, which is incontact with the second semiconductor layer, a silicon oxide film ispreferably used. This is because the trap level at the interface betweenthe second semiconductor layer and the second insulating film islowered.

When the second insulating film is in contact with Mo, a silicon oxidefilm is preferably used for a portion of the second insulating film incontact with Mo. This is because the silicon oxide film does not oxidizeMo.

A second conductive layer (conductive layers 4211 and 4212) is formedover the second insulating film. The conductive layer 4211 includes aportion functioning as a gate electrode of the transistor 4220. Theconductive layer 4212 functions as a second electrode of the capacitor4221 or a wiring. As the second conductive layer, an element such as Ti,Mo, Ta, Cr, W, Al, Nd, Cu, Ag, Au, Pt, Si, Zn, Fe, Ba, or Ge, or analloy of these elements can be used. Alternatively, a stacked layer ofthese elements (including the alloy thereof) can be used.

Note that in steps after forming the second conductive layer, variousinsulating films or various conductive films may be formed.

FIG. 51 shows cross-sectional structures of an inversely staggered(bottom gate) transistor and a capacitor. In particular, the transistorshown in FIG. 51 has a channel-etched structure.

A first insulating film (an insulating film 4302) is formed over anentire surface of a substrate 4301. The first insulating film canprevent impurities from the substrate from adversely affecting asemiconductor layer and changing properties of a transistor. That is,the first insulating film functions as a base film Thus, a transistorwith high reliability can be formed. As the first insulating film, asingle layer or a stacked layer of a silicon oxide film, a siliconnitride film, a silicon oxynitride film (SiO_(x)N_(y)), or the like canbe used.

Note that the first insulating film is not necessarily formed. When thefirst insulating film is not formed, reduction in the number of stepsand reduction in manufacturing cost can be realized. Further, since thestructure can be simplified, the yield can be improved.

A first conductive layer (conductive layers 4303 and 4304) is formedover the first insulating film. The conductive layer 4303 includes aportion functioning as a gate electrode of a transistor 4320. Theconductive layer 4304 includes a portion functioning as a firstelectrode of a capacitor 4321. As the first conductive layer, an elementsuch as Ti, Mo, Ta, Cr, W, Al, Nd, Cu, Ag, Au, Pt, Si, Zn, Fe, Ba, orGe, or an alloy of these elements can be used. Alternatively, a stackedlayer of these elements (including the alloy thereof) can be used.

A second insulating film (an insulating film 4305) is formed to cover atleast the first conductive layer. The second insulating film functionsas a gate insulating film. As the second insulating film, a single layeror a stacked layer of a silicon oxide film, a silicon nitride film, asilicon oxynitride film (SiO_(x)N_(y)), or the like can be used.

Note that for a portion of the second insulating film, which is incontact with the semiconductor layer, a silicon oxide film is preferablyused. This is because the trap level at the interface between thesemiconductor layer and the second insulating film is lowered.

When the second insulating film is in contact with Mo, a silicon oxidefilm is preferably used for a portion of the second insulating film incontact with Mo. This is because the silicon oxide film does not oxidizeMo.

A first semiconductor layer (a semiconductor layer 4306) is formed inpart of a portion over the second insulating film, which overlaps withthe first conductive layer, by a photolithography method, an inkjetmethod, a printing method, or the like. Part of the semiconductor layer4306 extends to a portion over the second insulating film, which doesnot overlap with the first conductive layer. The semiconductor layer4306 includes a portion functioning as a channel region of thetransistor 4320. As the semiconductor layer 4306, a semiconductor layerhaving no crystallinity such as an amorphous silicon (a-Si:H) layer, asemiconductor layer such as a microcrystalline semiconductor (μ-Si:H)layer, or the like can be used.

A second semiconductor layer (semiconductor layers 4307 and 4308) isformed over part of the first semiconductor layer. The semiconductorlayer 4307 includes a portion functioning as one of a source region anda drain region. The semiconductor layer 4308 includes a portionfunctioning as the other of the source electrode and the drainelectrode. As the second semiconductor layer, silicon containingphosphorus or the like can be used, for example.

A second conductive layer (conductive layers 4309, 4310, and 4311) isformed over the second semiconductor layer and the second insulatingfilm. The conductive layer 4309 includes a portion functioning as one ofthe source electrode and the drain electrode of the transistor 4320. Theconductive layer 4310 includes a portion functioning as the other of thesource electrode and the drain electrode of the transistor 4320. Theconductive layer 4311 includes a portion functioning as a secondelectrode of the capacitor 4321. As the second conductive layer, anelement such as Ti, Mo, Ta, Cr, W, Al, Nd, Cu, Ag, Au, Pt, Si, Zn, Fe,Ba, or Ge, or an alloy of these elements can be used. Alternatively, astacked layer of these elements (including the alloy thereof) can beused.

Note that in steps after forming the second conductive layer, variousinsulating films or various conductive films may be formed.

Here, an example of a step which is characteristic of the channel-etchedtype transistor is described. The first semiconductor layer and thesecond semiconductor layer can be formed using the same mask.Specifically, the first semiconductor layer and the second semiconductorlayer are continuously formed. Further, the first semiconductor layerand the second semiconductor layer are formed using the same mask.

Another example of a step which is characteristic of the channel-etchedtype transistor is described. The channel region of the transistor canbe formed without using an additional mask. Specifically, after thesecond conductive layer is faulted, part of the second semiconductorlayer is removed using the second conductive layer as a mask.Alternatively, part of the second semiconductor layer is removed byusing the same mask as the second conductive layer. The firstsemiconductor layer below the removed second semiconductor layer servesas the channel region of the transistor.

FIG. 52 shows cross-sectional structures of an inversely staggered(bottom gate) transistor and a capacitor. In particular, the transistorshown in FIG. 52 has a channel protection (channel stop) structure.

A first insulating film (an insulating film 4402) is formed over anentire surface of a substrate 4401. The first insulating film canprevent impurities from the substrate from adversely affecting asemiconductor layer and changing properties of a transistor. That is,the first insulating film functions as a base film. Thus, a transistorwith high reliability can be formed. As the first insulating film, asingle layer or a stacked layer of a silicon oxide film, a siliconnitride film, a silicon oxynitride film (SiO_(x)N_(y)), or the like canbe used.

Note that the first insulating film is not necessarily formed. When thefirst insulating film is not formed, reduction in the number of stepsand reduction in manufacturing cost can be realized. Further, since thestructure can be simplified, the yield can be improved.

A first conductive layer (conductive layers 4403 and 4404) is formedover the first insulating film. The conductive layer 4403 includes aportion functioning as a gate electrode of a transistor 4420. Theconductive layer 4404 includes a portion functioning as a firstelectrode of a capacitor 4421. As the first conductive layer, an elementsuch as Ti, Mo, Ta, Cr, W, Al, Nd, Cu, Ag, Au, Pt, Si, Zn, Fe, Ba, orGe, or an alloy of these elements can be used. Alternately, a stackedlayer of these elements (including the alloy thereof) can be used.

A second insulating film (an insulating film 4405) is formed to cover atleast the first conductive layer. The second insulating film functionsas a gate insulating film. As the second insulating film, a single layeror a stacked layer of a silicon oxide film, a silicon nitride film, asilicon oxynitride film (SiO_(x)N_(y)), or the like can be used.

Note that for a portion of the second insulating film, which is incontact with the semiconductor layer, a silicon oxide film is preferablyused. This is because the trap level at the interface between thesemiconductor layer and the second insulating film is lowered.

When the second insulating film is in contact with Mo, a silicon oxidefilm is preferably used for a portion of the second insulating film incontact with Mo. This is because the silicon oxide film does not oxidizeMo.

A first semiconductor layer (a semiconductor layer 4406) is formed inpart of a portion over the second insulating film, which overlaps withthe first conductive layer, by a photolithography method, an inkjetmethod, a printing method, or the like. Part of the semiconductor layer4406 extends to a portion over the second insulating film, which doesnot overlap with the first conductive layer. The semiconductor layer4406 includes a portion functioning as a channel region of thetransistor 4420. As the semiconductor layer 4406, a semiconductor layerhaving no crystallinity such as an amorphous silicon (a-Si:H) layer, asemiconductor layer such as a microcrystalline semiconductor (μ-Si:H)layer, or the like can be used.

A third insulating film (an insulating film 4412) is formed over part ofthe first semiconductor layer. The insulating film 4412 prevents thechannel region of the transistor 4420 from being removed by etching.That is, the insulating film 4412 functions as a channel protection film(a channel stop film). As the third insulating film, a single layer or astacked layer of a silicon oxide film, a silicon nitride film, a siliconoxynitride film (SiO_(x)N_(y)), or the like can be used.

A second semiconductor layer (semiconductor layers 4407 and 4408) isformed over part of the first semiconductor layer and part of the thirdinsulating film. The semiconductor layer 4407 includes a portionfunctioning as one of a source region and a drain region. Thesemiconductor layer 4408 includes a portion functioning as the other ofthe source region and the drain region. As the second semiconductorlayer, silicon containing phosphorus or the like can be used forexample.

A second conductive layer (conductive layers 4409, 4410, and 4411) isformed over the second semiconductor layer. The conductive layer 4409includes a portion functioning as one of the source electrode and thedrain electrode of the transistor 4420. The conductive layer 4410includes a portion functioning as the other of the source electrode andthe drain electrode of the transistor 4420. The conductive layer 4411includes a portion functioning as a second electrode of the capacitor4421. As the second conductive layer, an element such as Ti, Mo, Ta, Cr,W, Al, Nd, Cu, Ag, Au, Pt, Si, Zn, Fe, Ba, or Ge, or an alloy of theseelements can be used. Alternately, a stacked layer of these elements(including the alloy thereof) can be used.

Note that in steps after forming the second conductive layer, variousinsulating films or various conductive films may be formed.

Here, an example of a step which is characteristic of the channelprotection type transistor is described. The first semiconductor layer,the second semiconductor layer, and the second conductive layer can beformed using the same mask. At the same time, the channel region can beformed. Specifically, the first semiconductor layer is formed, and then,the third insulating film (i.e., the channel protection film or thechannel stop film) is patterned using a mask. Next, the secondsemiconductor layer and the second conductive layer are continuouslyformed. Then, after the second conductive layer is formed, the firstsemiconductor layer, the second semiconductor layer, and the secondconductive film are patterned using the same mask. Note that part of thefirst semiconductor layer below the third insulating film is protectedby the third insulating film, and thus is not removed by etching. Thispart (a part of the first semiconductor layer over which the thirdinsulating film is formed) serves as the channel region.

Note that although this embodiment mode is described with reference tovarious drawings, the contents (or part of the contents) described ineach drawing can be freely applied to, combined with, or replaced withthe contents (or part of the contents) described in another drawing.Further, much more drawings can be formed by combining each part withanother part in the above-described drawings.

Similarly, the contents (or part of the contents) described in eachdrawing in this embodiment mode can be freely applied to, combined with,or replaced with the contents (or part of the contents) described in adrawing in another embodiment mode or embodiment. Further, much moredrawings can be formed by combining each part in each drawing in thisembodiment mode with part of another embodiment mode or embodiment.

Note that this embodiment mode shows examples of embodying, slightlytransforming, partially modifying, improving, describing in detail, orapplying the contents (or part of the contents) described in otherembodiment modes and embodiments, an example of related part thereof, orthe like. Therefore, the contents described in other embodiment modesand embodiments can be freely applied to, combined with, or replacedwith this embodiment mode.

[Embodiment Mode 12]

In this embodiment mode, examples of electronic devices according to thepresent invention are described.

FIG. 53 shows one mode of a display panel module in which a displaypanel 4501 and a circuit board 4502 are combined.

As shown in FIG. 53, the display panel 4501 includes a pixel portion4503, a scan line driver circuit 4504, and a signal line driver circuit4505. The circuit board 4502 is provided with a control circuit 4506, asignal dividing circuit 4507, and the like, for example. Note that thedisplay panel 4501 and the circuit board 4502 are connected by aconnection wiring 4508. As the connection wiring 4508, an FPC or thelike can be used.

In the display panel 4501, the pixel portion and part of peripheraldriver circuits (a driver circuit having a low operation frequency amonga plurality of driver circuits) may be formed using transistors over asubstrate, and another peripheral driver circuit (a driver circuithaving a high operation frequency among the plurality of drivercircuits) may be formed on an IC chip. The IC chip may be mounted on thedisplay panel 4501 by COG (chip on glass). The IC chip may alternativelybe connected to a glass substrate by using TAB (tape automated bonding)or a printed circuit board. Further, all of the peripheral drivercircuits may be formed on an IC chip and the IC chip may be mounted onthe display panel by COG or the like.

Note that the pixels described in any of the above-described embodimentmodes are used for the pixel portion. According to the presentinvention, viewing angle characteristics can be enhanced. Further, costreduction can be achieved by using transistors with the sameconductivity type as transistors included in the pixel portion or usingan amorphous semiconductor for semiconductor layers of the transistors.

A TV receiver can be completed with such a display module. FIG. 54 is ablock diagram showing a main structure of a TV receiver. A tuner 4601receives a video signal and an audio signal. The video signal isprocessed by a video signal amplifier circuit 4602, a video signalprocessing circuit 4603 for converting a signal output from the videosignal amplifier circuit 4602 into a color signal corresponding to eachcolor of red, green, and blue, and a control circuit 4506 for convertingthe video signal into a signal which meets input specifications of adriver circuit. The control circuit 4506 outputs signals to a scan lineside and a signal line side. In the case of performing a digital drive,a structure can be used in which the signal dividing circuit 4507 isprovided on the signal line side to supply an input digital signaldivided into m (m is a positive integer) pieces.

The audio signal among the signals received by the tuner 4601 istransmitted to an audio signal amplifier circuit 4604, and an output ofthe audio signal amplifier circuit 4604 is supplied to a speaker 4606through an audio signal processing circuit 4605. A control circuit 4607receives control information of a receiving station (receptionfrequency) or sound volume from an input portion 4608, and transmitssignals to the tuner 4601 and the audio signal processing circuit 4605.

FIG. 55A shows a TV receiver incorporated with a display panel modulewhich is different from that of FIG. 54. In FIG. 55A, a display screen4702 stored in a housing 4701 is formed by using the display panelmodule. Note that speakers 4703, operation switches 4704, or the likemay be provided as appropriate.

FIG. 55B shows a TV receiver, only a display of which can be carriedwirelessly. A battery and a signal receiver are incorporated in ahousing 4712. The battery drives a display portion 4713 and speakerportions 4717. The battery can be repeatedly charged with a charger4710. The charger 4710 can transmit and receive a video signal and cantransmit the video signal to the signal receiver of the display. Thehousing 4712 is controlled by operation keys 4716. Alternatively, thedevice shown in FIG. 55B may be an image audio two-way communicationdevice which can transmit a signal to the charger 4710 from the housing4712 by operating the operation keys 4716. Still alternatively, thedevice shown in FIG. 55B may be a general-purpose remote control devicewhich can transmit a signal to the charger 4710 from the housing 4712 byoperating the operation keys 4716, and can control communication ofanother electronic device when the electronic device is made to receivea signal which can be transmitted from the charger 4710. The presentinvention can be applied to the display portion 4713.

FIG. 56A shows a module in which a display panel 4801 and a printedwiring board 4802 are combined. The display panel 4801 is provided witha pixel portion 4803 including a plurality of pixels, a first scan linedriver circuit 4804, a second scan line driver circuit 4805, and asignal line driver circuit 4806 which supplies a video signal to aselected pixel.

The printed wiring board 4802 is provided with a controller 4807, acentral processing unit (CPU) 4808, a memory 4809, a power supplycircuit 4810, an audio processing circuit 4811, a transmitting/receivingcircuit 4812, and the like. The printed wiring board 4802 and thedisplay panel 4801 are connected by a flexible printed circuit (FPC)4813. The flexible printed circuit (FPC) 4813 may be provided with astorage capacitor, a buffer circuit, or the like to prevent noise onpower supply voltage or a signal, and increase in rise time of a signal.Note that the controller 4807, the audio processing circuit 4811, thememory 4809, the central processing unit (CPU) 4808, the power supplycircuit 4810, and the like can be mounted on the display panel 4801 byusing a COG (chip on glass) method. When a COG method is used, the sizeof the printed wiring board 4802 can be reduced.

Various control signals are input and output through an interface (I/F)portion 4814 provided for the printed wiring board 4802. In addition, anantenna port 4815 for transmitting and receiving a signal to/from anantenna is provided for the printed wiring board 4802.

FIG. 5613 is a block diagram of the module shown in FIG. 56A. The moduleincludes a VRAM 4816, a DRAM 4817, a flash memory 4818, and the like asthe memory 4809. The VRAM 4816 stores data on an image displayed on thepanel. The DRAM 4817 stores video data or audio data. The flash memory4818 stores various programs.

The power supply circuit 4810 supplies electric power for operating thedisplay panel 4801, the controller 4807, the central processing unit(CPU) 4808, the audio processing circuit 4811, the memory 4809, and thetransmitting/receiving circuit 4812. Note that depending on panelspecifications, the power supply circuit 4810 is provided with a currentsource in some cases.

The central processing unit (CPU) 4808 includes a control signalgeneration circuit 4820, a decoder 4821, a register 4822, an arithmeticcircuit 4823, a RAM 4824, an interface (I/F) portion 4819 for thecentral processing unit (CPU) 4808, and the like. Various signals whichare input to the central processing unit (CPU) 4808 through theinterface (I/F) portion 4819 are once stored in the register 4822, andthen input to the arithmetic circuit 4823, the decoder 4821, and thelike. The arithmetic circuit 4823 performs operation based on the inputsignal to designate a location to which various instructions are sent.On the other hand, the signal input to the decoder 4821 is decoded andinput to the control signal generation circuit 4820. The control signalgeneration circuit 4820 generates a signal including variousinstructions based on the input signal, and transmits the signal tolocations designated by the arithmetic circuit 4823, specifically, thememory 4809, the transmitting/receiving circuit 4812, the audioprocessing circuit 4811, the controller 4807, and the like.

The memory 4809, the transmitting/receiving circuit 4812, the audioprocessing circuit 4811, and the controller 4807 operate in accordancewith respective instructions. Operations thereof are briefly describedbelow.

A signal input from an input means 4825 is transmitted to the centralprocessing unit (CPU) 4808 mounted on the printed wiring board 4802through the interface (I/F) portion 4814. The control signal generationcircuit 4820 converts image data stored in the VRAM 4816 into apredetermined format based on the signal transmitted from the inputmeans 4825 such as a pointing device or a keyboard, and transmits theconverted data to the controller 4807.

The controller 4807 performs data processing of the signal including theimage data transmitted from the central processing unit (CPU) 4808 inaccordance with the panel specifications, and supplies the signal to thedisplay panel 4801. The controller 4807 generates an Hsync signal, aVsync signal, a clock signal CLK, alternating voltage (AC Cont), and aswitching signal L/R based on power supply voltage input from the powersupply circuit 4810 or various signals input from the central processingunit (CPU) 4808, and supplies the signals to the display panel 4801.

The transmitting/receiving circuit 4812 processes a signal which istransmitted and received as a radio wave by an antenna 4828.Specifically, the transmitting/receiving circuit 4812 may include ahigh-frequency circuit such as an isolator, a band pass filter, a VCO(voltage controlled oscillator), an LPF (low pass filter), a coupler, ora balun. Among signals transmitted and received by thetransmitting/receiving circuit 4812, a signal including audioinformation is transmitted to the audio processing circuit 4811 inaccordance with an instruction from the central processing unit (CPU)4808.

The signal including the audio information, which is transmitted inaccordance with the instruction from the central processing unit (CPU)4808, is demodulated into an audio signal by the audio processingcircuit 4811 and is transmitted to a speaker 4827. An audio signaltransmitted from a microphone 4826 is modulated by the audio processingcircuit 4811 and is transmitted to the transmitting/receiving circuit4812 in accordance with an instruction from the central processing unit(CPU) 4808.

The controller 4807, the central processing unit (CPU) 4808, the powersupply circuit 4810, the audio processing circuit 4811, and the memory4809 can be mounted as a package of this embodiment mode.

Needless to say, the present invention is not limited to the TVreceiver, and can be applied to various uses particularly as a largedisplay medium such as an information display board at a train station,an airport, or the like, or an advertisement display board on thestreet, as well as a monitor of a personal computer.

Next, a structural example of a mobile phone according to the presentinvention is described with reference to FIG. 57.

A display panel 4901 is incorporated in a housing 4930 so as to bedetachable. The shape and the size of the housing 4930 can be changed asappropriate in accordance with the size of the display panel 4901. Thehousing 4930 to which the display panel 4901 is fixed is fitted into aprinted circuit board 4931 and is assembled as a module.

The display panel 4901 is connected to the printed circuit board 4931through an FPC 4913. The printed circuit board 4931 is provided with aspeaker 4932, a microphone 4933, a transmitting/receiving circuit 4934,and a signal processing circuit 4935 including a CPU, a controller, andthe like. Such a module, an input means 4936, and a battery 4937 arecombined and stored in a housing 4939. A pixel portion of the displaypanel 4901 is provided so as to be seen from an opening window formed inthe housing 4939.

In the display panel 4901, the pixel portion and part of peripheraldriver circuits (a driver circuit having a low operation frequency amonga plurality of driver circuits) may be formed over a substrate by usingtransistors, and another part of the peripheral driver circuits (adriver circuit having a high operation frequency among the plurality ofdriver circuits) may be formed over an IC chip. Then, the IC chip may bemounted on the display panel 4901 by COG (chip on glass). Alternatively,the IC chip may be connected to a glass substrate by using TAB (tapeautomated bonding) or a printed circuit board. With such a structure,power consumption of the mobile phone (the display panel is alsopossible) can be reduced, and operation time of the mobile phone percharge can be extended. Further, reduction in cost of the mobile phonecan be realized.

The mobile phone shown in FIG. 57 has various functions such as afunction of displaying a variety of information (e.g., a still image, amoving image, and a text image); a function of displaying a calendar, adate, time, or the like on a display portion; a function of operating orediting the information displayed on the display portion; a function ofcontrolling processing by a variety of software (programs); a wirelesscommunication function; a function of communicating with another mobilephone, a fixed phone, or an audio communication device by using thewireless communication function; a function of connecting with a varietyof computer networks by using the wireless communication function; afunction of transmitting or receiving a variety of data by using thewireless communication function; a function of operating a vibrator inaccordance with incoming call, reception of data, or an alarm; and afunction of generating a sound in accordance with incoming call,reception of data, or an alarm. Note that functions of the mobile phoneshown in FIG. 57 are not limited to them, and the mobile phone can havevarious functions.

In a mobile phone shown in FIG. 58, a main body (A) 5001 which isprovided with operation switches 5004, a microphone 5005, and the likeis connected to a main body (B) 5002 which is provided with a displaypanel (A) 5008, a display panel (B) 5009, a speaker 5006, and the likeby using a hinge 5010 so that the mobile phone can be opened and closed.The display panel (A) 5008 and the display panel (B) 5009 are stored ina housing 5003 of the main body (B) 5002 together with a circuit board5007. Each of pixel portions of the display panel (A) 5008 and thedisplay panel (B) 5009 is provided so as to be seen from an openingwindow formed in the housing 5003.

Specifications of the display panel (A) 5008 and the display panel (B)5009, such as the number of pixels, can be set as appropriate inaccordance with functions of a mobile phone 5000. For example, thedisplay panel (A) 5008 can be used as a main screen and the displaypanel (B) 5009 can be used as a sub-screen.

Each of the mobile phones of this embodiment mode can be changed invarious modes depending on functions or applications thereof. Forexample, it may be a camera-equipped mobile phone by incorporating animaging element in a portion of the hinge 5010. When the operationswitches 5004, the display panel (A) 5008, and the display panel (B)5009 are stored in one housing, the above-described advantageous effectscan be obtained. Further, similar advantageous effects can be obtainedwhen the structure of this embodiment mode is applied to an informationdisplay terminal provided with a plurality of display portions.

The mobile phone shown in FIG. 58 has various functions such as afunction of displaying a variety of information (e.g., a still image, amoving image, and a text image); a function of displaying a calendar, adate, time, or the like on a display portion; a function of operating orediting the information displayed on the display portion; a function ofcontrolling processing by a variety of software (programs); a wirelesscommunication function; a function of communicating with another mobilephone, a fixed phone, or an audio communication device by using thewireless communication function; a function of connecting with a varietyof computer networks by using the wireless communication function; afunction of transmitting or receiving a variety of data by using thewireless communication function; a function of operating a vibrator inaccordance with incoming call, reception of data, or an alarm; and afunction of generating a sound in accordance with incoming call,reception of data, or an alarm. Note that functions of the mobile phoneshown in FIG. 58 are not limited to them, and the mobile phone can havevarious functions.

The present invention can be applied to various electronic devices.Specifically, the present invention can be applied to display portionsof electronic devices. Examples of such electronic devices are camerassuch as a video camera and a digital camera, a goggle-type display, anavigation system, an audio reproducing device (e.g., a car audiocomponent or an audio component), a computer, a game machine, a portableinformation terminal (e.g., a mobile computer, a mobile phone, a mobilegame machine, or an electronic book), an image reproducing deviceprovided with a recording medium (specifically, a device whichreproduces a recording medium such as a digital versatile disc (DVD) andhas a display for displaying a reproduced image), and the like.

FIG. 59A shows a display, which includes a housing 5111, a support base5112, a display portion 5113, and the like. The display shown in FIG.59A has a function of displaying a variety of information (e.g., a stillimage, a moving image, and a text image) on the display portion. Notethat the function of the display shown in FIG. 59A is not limited tothis function, and the display can have various functions.

FIG. 59B shows a camera, which includes a main body 5121, a displayportion 5122, an image receiving portion 5123, operation keys 5124, anexternal connection port 5125, a shutter button 5126, and the like. Thecamera shown in FIG. 59B has a function of photographing a still imageand a moving image; a function of automatically correcting thephotographed image (the still image or the moving image); a function ofstoring the photographed image in a recording medium (provided outsideor incorporated in the camera); and a function of displaying thephotographed image on the display portion. Note that the functions ofthe camera shown in FIG. 59B are not limited to these functions, and thecamera can have various functions.

FIG. 59C shows a computer, which includes a main body 5131, a housing5132, a display portion 5133, a keyboard 5134, an external connectionport 5135, a pointing device 5136, and the like. The computer shown inFIG. 59C has a function of displaying a variety of information (e.g., astill image, a moving image, and a text image) on the display portion; afunction of controlling processing by a variety of software (programs);a communication function such as wireless communication or wirecommunication; a function of connecting to various computer networks byusing the communication function; and a function of transmitting orreceiving a variety of data by using the communication function. Notethat the functions of the computer shown in FIG. 59C are not limited tothese functions, and the computer can have various functions.

FIG. 59D shows a mobile computer, which includes a main body 5141, adisplay portion 5142, a switch 5143, operation keys, 5144, an infraredport 5145, and the like. The mobile computer shown in FIG. 59D has afunction of displaying a variety of information (e.g., a still image, amoving image, and a text image) on the display portion; a touch panelfunction on the display portion; a function of displaying a calendar, adate, time, and the like on the display portion; a function ofcontrolling processing by a variety of software (programs); a wirelesscommunication function; a function of connecting to various computernetworks by using the wireless communication function; and a function oftransmitting or receiving a variety of data by using the wirelesscommunication function. Note that the functions of the mobile computershown in FIG. 59D are not limited to these functions, and the mobilecomputer can have various functions.

FIG. 59E shows a portable image reproducing device provided with arecording medium (e.g., a DVD reproducing device), which includes a mainbody 5151, a housing 5152, a display portion A 5153, a display portion B5154, a recording medium (e.g., DVD) reading portion 5155, operationkeys 5156, a speaker portion 5157, and the like. The display portion A5153 can mainly display image information, and the display portion B5154 can mainly display text information.

FIG. 59F shows a goggle-type display, which includes a main body 5161, adisplay portion 5162, an earphone 5163, a support portion 5164, and thelike. The goggle-type display shown in FIG. 59F has a function ofdisplaying an image (e.g., a still image, a moving image, or a textimage) which is externally obtained on the display portion. Note thatthe functions of the goggle-type display shown in FIG. 59F are notlimited to these functions, and the goggle-type display can have variousfunctions.

FIG. 59G shows a portable game machine, which includes a housing 5171, adisplay portion 5172, speaker portions 5173, operation keys 5174, arecording medium insert portion 5175, and the like. A portable gamemachine, in which the display device of the present invention is usedfor the display portion 5172, can display vivid colors. The portablegame machine shown in FIG. 59G has a function of reading a program ordata stored in the recording medium to display it on the displayportion, and a function of sharing information with another portablegame machine by wireless communication. Note that the functions of theportable game machine shown in FIG. 590 are not limited to thesefunctions, and the portable game machine can have various functions.

FIG. 59H shows a digital camera having a television reception function,which includes a main body 5181, a display portion 5182, operation keys5183, a speaker 5184, a shutter button 5185, an image receiving portion5186, an antenna 5187, and the like. The digital camera having thetelevision reception function, which is shown in FIG. 59H, has afunction of photographing a still image and a moving image; a functionof automatically correcting the photographed image; a function ofobtaining a variety of information from the antenna; a function ofstoring the photographed image or the information obtained from theantenna; and a function of displaying the photographed image or theinformation obtained from the antenna on the display portion. Note thatthe functions of the digital camera having the television receptionfunction, which is shown in FIG. 59H, are not limited to thesefunctions, and the digital camera having the television receptionfunction can have various functions.

As shown in FIGS. 59A to 59E, the electronic appliances in accordancewith the present invention have display portions for displaying someinformation. The electronic appliance of the present invention has lowpower consumption and can drive with a battery for a long time, becausein the case where pieces of data overlap with each other, theoverlapping data is stored in a memory and thus the frequency of circuitoperations can be reduced. Next, application examples of the displaydevices of the present invention will be described.

FIG. 60 shows an example in which the display device in accordance withthe present invention is incorporated in a structure. FIG. 60 shows thestructure which includes a housing 5200, a display panel 5201, a speakerportion 5202, and the like. Note that a reference numeral 5203 denotes aremote controller for operating the display panel 5201.

The pixel described in any of the above embodiment modes is used for thedisplay panel 5201. By using the present invention, a high-definitiondisplay panel which is viewable at wider angles can be obtained. Notethat further, cost reduction can be achieved by using transistors withthe same conductivity type as transistors included in the pixel portionor using an amorphous semiconductor for semiconductor layers of thetransistors.

Since the display device shown in FIG. 60 is incorporated in thestructure, the display device shown in FIG. 60 can be provided withoutrequiring a wide space.

FIG. 61 shows another example in which the display device in accordancewith the present invention is incorporated in a structure. A displaypanel 5301 is incorporated in a prefabricated bath unit 5302, so that abather can view the display panel 5301 while taking a bath. Informationcan be displayed on the display panel 5301 by an operation of thebather. Therefore, the display panel 5301 can be used for advertisementor an amusement means.

The pixel described in any of the above embodiment modes is used for thedisplay panel 5301. By using the present invention, a high-definitiondisplay panel which is viewable at wider angles can be obtained. Notethat further, cost reduction can be achieved by using transistors withthe same conductivity type as transistors included in the pixel portionor using an amorphous semiconductor for semiconductor layers of thetransistors.

The position for providing the display device in accordance with thepresent invention is not limited to a sidewall of the prefabricated bathunit 5302 shown in FIG. 61, and the display device in accordance withthe present invention can be incorporated in various places. Forexample, the display device in accordance with the present invention canbe incorporated in part of a mirror or the bathtub itself. In addition,the shape of the display device may be a shape in accordance with themirror or the bathtub.

FIG. 62 shows another example in which the display device in accordancewith the present invention is incorporated in a structure. In FIG. 62,display panels 5402 are curved in accordance with curved surfaces ofcolumnar objects 5401. Here, the columnar objects 5401 are described astelephone poles.

The display panels 5402 shown in FIG. 62 are provided at a positionhigher than a human eye level. When the display panels 5402 are providedfor structures standing outside together in large numbers such astelephone poles, it is possible to provide information to theunspecified number of viewers through the display panels 5402.Therefore, the display panels are suitable for advertisement. Since thedisplay panels 5402 can easily display the same images by control fromoutside and can easily switch images instantly, extremely effectiveinformation display and advertising effects can be expected. Inaddition, by providing self-luminous display elements in the displaypanels 5402, the display panels 5402 can be effectively used as highlyvisible display media even at night. Further, by providing the displaypanels 5402 for the telephone poles, power supply means of the displaypanels 5402 can be easily secured. In an emergency such as a disaster,the display panels 5402 can be means for quickly transmitting preciseinformation to victims.

The pixel described in any of the above embodiment modes is used foreach of the display panels 5402. By using the present invention, ahigh-definition display panel which is viewable at wider angles can beobtained. Note that further, cost reduction can be achieved by usingtransistors with the same conductivity type as transistors included inthe pixel portion or using an amorphous semiconductor for semiconductorlayers of the transistors. Alternatively, an organic transistor providedover a film substrate may be used.

Note that although this embodiment describes the wall, the prefabricatedbath unit, and the columnar object as examples of the structure in whichthe display device in accordance with the present invention isincorporated, the display device in accordance with the presentinvention can be provided for various structures.

Next, an example is described in which the display device in accordancewith the present invention is incorporated in a moving object.

FIG. 63 shows an example in which the display device in accordance withthe present invention is incorporated in a car. A display panel 5502 isincorporated in a car body 5501 of the car and can display informationon an operation of the car or information input from inside or outsideof the car on an on-demand basis. Further, the display panel 5502 mayhave a navigation function.

The pixel described in any of the above embodiment modes is used for thedisplay panel 5502. By using the present invention, a high-definitiondisplay panel which is viewable at wider angles can be obtained. Notethat further, cost reduction can be achieved by using transistors withthe same conductivity type as transistors included in the pixel portionor using an amorphous semiconductor for semiconductor layers of thetransistors.

Note that the display device in accordance with the present inventioncan be provided in not only the car body 5501 shown in FIG. 63 but alsoin various positions. For example, the display device in accordance withthe present invention may be incorporated in a glass window, a door, asteering wheel, a shift lever, a seat, a room mirror, or the like. Atthis time, the shape of the display panel 5502 may be a shape inaccordance with the shape of an object in which the display panel 5502is provided.

FIGS. 64A and 64B each show an example in which the display device inaccordance with the present invention is incorporated in a train car.

FIG. 64A shows an example in which display panels 5602 are provided forglasses of a door 5601 of the train car. The display panels 5602 have anadvantage over conventional paper-based advertisement that labor costwhich is necessary for switching advertisement is not needed. Inaddition, since the display panels 5602 can instantly switch imagesdisplayed on a display portion by external signals, images on thedisplay panel can be switched as the type of train passengers changes inaccordance with different time periods. By changing images instantly inthis manner, a more effective advertising effect can be expected.

FIG. 64B shows an example in which display panels 5602 are provided forglass windows 5603 and a ceiling 5604 as well as the glasses of thedoors 5601 of the train car. Since the display device in accordance withthe present invention can be easily provided in a position in which thedisplay device is conventionally difficult to be provided, an effectiveadvertisement effect can be obtained. In addition, since the displaydevice in accordance with the present invention can instantly switchimages displayed on the display portion by external signals, cost andtime generated in advertisement switching can be reduced and moreflexible advertisement operation and information transmission can beperformed.

The pixel described in any of the above embodiment modes is used foreach of the display panels 5602 shown in FIGS. 64A and 64B. By using thepresent invention, a high-definition display panel which is viewable atwider angles can be obtained. Note that further, cost reduction can beachieved by using transistors with the same conductivity type astransistors included in the pixel portion or using an amorphoussemiconductor for semiconductor layers of the transistors.

Note that the position of the display device in accordance with thepresent invention is not limited to the above-described position and thedisplay device can be provided in various positions. For example, thedisplay device in accordance with the present invention may beincorporated in a hand strap, a seat, a handrail, a floor, or the like.At this time, the shape of the display panel 5602 may be a shape inaccordance with the shape of an object in which the display panel 5602is provided.

FIGS. 65A and 65B each show an example in which the display device inaccordance with the present invention is incorporated in a passengerairplane.

FIG. 65A shows a shape in use when a display panel 5702 is provided to aceiling 5701 above a seat of the passenger airplane. The display panel5702 is incorporated in the ceiling 5701 through a hinge portion 5703,and a passenger can view the display panel 5702 at a desired position bya telescopic motion of the hinge portion 5703. Information can bedisplayed on the display panel 5702 by an operation of the passenger.Therefore, the display panel 5702 can be used for advertisement or anamusement means. In addition, by storing the display panel 5702 on theceiling 5701 by folding the hinge portion 5703 as shown in FIG. 65B,safety during takeoff and landing can be secured. Note that the displaypanel 5702 can also be utilized as a medium and a guide light bylighting display elements of the display panel 5702 in an emergency.

Note that the pixel described in any of the above embodiment modes isused for each of the display panel 5702 shown in FIGS. 65A and 65B. Byusing the present invention, a high-definition display panel which isviewable at wider angles can be obtained. Note that further, costreduction can be achieved by using transistors with the sameconductivity type as transistors included in the pixel portion or usingan amorphous semiconductor for semiconductor layers of the transistors.

Note that the display device in accordance with the present inventioncan be incorporated in not only the ceiling 5701 shown in FIGS. 65A and65B but also in various positions. For example, the display device inaccordance with the present invention may be incorporated in a seat, atable, an armrest, a window, or the like. In addition, a large displaypanel which can be viewed simultaneously by a large number of people maybe provided in a wall of an airframe. At this time, the shape of thedisplay panel 5702 may be a shape in accordance with the shape of anobject in which the display panel 5702 is provided.

Although this embodiment mode describes the train car body, the carbody, and the airplane body as examples of moving objects, the presentinvention is not limited to them, and can be applied to a motorbike, afour-wheeled vehicle (including a car, a bus, and the like), a train(including a monorail, a rail, and the like), a vessel, and the like. Inaddition, since display on display panels in a moving object can beswitched instantly by external signals, the display device in accordancewith the present invention can be used for an advertisement displayboard for an unspecified number of customers, or an information displayboard in an emergency by providing the display device in accordance withthe present invention in the moving object.

Note that although this embodiment mode is described with reference tovarious drawings, the contents (or part of the contents) described ineach drawing can be freely applied to, combined with, or replaced withthe contents (or part of the contents) described in another drawing.Further, much more drawings can be formed by combining each part withanother part in the above-described drawings.

Similarly, the contents (or part of the contents) described in eachdrawing in this embodiment mode can be freely applied to, combined with,or replaced with the contents (or part of the contents) described in adrawing in another embodiment mode. Further, much more drawings can beformed by combining each part in each drawing in this embodiment modewith part of another embodiment mode.

Note that this embodiment mode shows examples of embodying, slightlytransforming, partially modifying, improving, describing in detail, orapplying the contents (or part of the contents) described in otherembodiment modes, an example of related part thereof, or the like.Therefore, the contents described in other embodiment modes can befreely applied to, combined with, or replaced with this embodiment mode.

This application is based on Japanese Patent Application serial no.2007432172 filed in Japan Patent Office on May 17, 2007, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. A semiconductor device comprising a firstsub-pixel and a second sub-pixel, the semiconductor device comprising: afirst line; a second line crossing the first line; a first thin filmtransistor wherein a gate of the first thin film transistor iselectrically connected to the first line, one of a source and a drain ofthe first thin film transistor is electrically connected to the secondline; a first pixel electrode of the first sub-pixel, wherein the firstpixel electrode is electrically connected to the other of the source andthe drain of the first thin film transistor; a first capacitor wherein afirst electrode of the first capacitor is electrically connected to thefirst pixel electrode; a second thin film transistor wherein a gate ofthe second thin film transistor is electrically connected to the firstline, one of a source and a drain of the second thin film transistor iselectrically connected to the second line; a second pixel electrode ofthe second sub-pixel, wherein the second pixel electrode is electricallyconnected to the other of the source and the drain of the second thinfilm transistor; a second capacitor wherein a first electrode of thesecond capacitor is electrically connected to the second pixelelectrode; a switch wherein the one of the source and the drain of thefirst thin film transistor is electrically connected to the second linethrough the switch; and a third pixel electrode wherein the third pixelelectrode is electrically connected to the one of the source and thedrain of the first thin film transistor, wherein the first sub-pixel islocated on a first side of the first line and the second sub-pixel islocated on a second side of the first line, the first side beingopposite to the second side, and wherein each of the first thin filmtransistor and the second thin film transistor comprises a channel whichcomprises indium, zinc and oxygen.
 2. The semiconductor device accordingto claim 1, wherein the semiconductor device is a display device.
 3. Amodule comprising: a circuit board; and a display panel electricallyconnected to the circuit board, the display panel comprising, a firstsub-pixel and a second sub-pixel, the display device comprising: a firstline; a second line crossing the first line; a first thin filmtransistor wherein a gate of the first thin film transistor iselectrically connected to the first line, one of a source and a drain ofthe drain of the first thin film transistor is electrically connected tothe second line; a first pixel electrode of the first sub-pixel, whereinthe first pixel electrode is electrically connected to the other of thesource and the drain of the first thin film transistor; a firstcapacitor wherein a first electrode of the first capacitor iselectrically connected to the first pixel electrode; a second thin filmtransistor wherein a gate of the second thin film transistor iselectrically connected to the first line, one of a source and a drain ofthe second thin film transistor is electrically connected to the secondline; a second pixel electrode of the sub-pixel, wherein the secondpixel electrode is electrically connected to the other of the source andthe drain of the second thin film transistor; a second capacitor whereina first electrode of the second capacitor is electrically connected tothe second pixel electrode; a switch wherein the one of the source andthe drain of the first thin film transistor is electrically connected tothe second line through the switch; and a third pixel electrode whereinthe third pixel electrode is electrically connected to the one of thesource and the drain of the first thin film transistor, wherein thefirst sub-pixel is located on a first side of the first line and thesecond sub-pixel is located on a second side of the first line, thefirst side being opposite to the second side, and wherein each of thefirst thin film transistor and the second thin film transistor comprisesa channel which comprises indium, zinc and oxygen.
 4. The semiconductordevice according to claim 3, wherein the semiconductor device is adisplay device.